Exegesis:
en hois episunachtheison tōn muriadōn tou ochlou ‘meanwhile, when the (usual) thousands of people gathered.’ en hois, lit. ‘during which,’ with relative pronoun, has demonstrative force and is equivalent to en toutois lit. ‘during these things,’ i.e. the things reported in 11.37-54.
episunagō ‘to bring together,’ ‘to gather,’ here in the passive with intransitive force.
murias ‘myriad,’ ‘then thousand,’ here in the plural, of a very large number without exact definition. The article before muriadōn points to what is usual. The phrase tōn muriadōn tou ochlou is hyperbolical (as in Acts 21.20).
hōste katapatein allēlous ‘so that they actually were treading upon one another.’ hōste with following infinitive indicates the actual result.
katapateō (also 8.5) ‘to tread upon,’ ‘to trample’ (8.5), here of treading upon each other’s feet, or, of trampling under foot, preferably the latter.
ērxato legein pros tous mathētas autou prōton ‘he began to say to his disciples first of all.’ For ērxato cf. on 4.21. prōton does not refer to order but to degree: the crowds are listening too.
prosechete heautois apo tēs zumēs, hēitis estin hupokrisis, tōn Pharisaiōn ‘be on your guard against the yeast, that means, the hypocrisy of the Pharisees.’
prosechō lit. ‘to turn one’s mind to,’ with dative of the reflexive pronoun, ‘to be on one’s guard,’ here followed by apo as indication of that against which one is on one’s guard.
zumē (also 13.21) ‘yeast,’ ‘leaven,’ a substance to produce fermentation, here in a figurative sense, explained by the subsequent relative clause.
hupokrisis ‘hypocrisy,’ ‘outward pretense.’
Translation:
The structure of the introductory clause may have to be recast, e.g. ‘meanwhile (or, while all this happened) the crowd was gathering; there were so many thousands (or, so very many people) that they … Then Jesus….’
Had gathered together, or, ‘had come together (where Jesus was).’
Trod upon one another, or, ‘trampled each other under foot,’ ‘crowded each other out’ (Nieuwe Vertaling), ‘pushed/elbowed each other aside,’ or any other idiom the receptor language uses for an accumulation of people in a crowded place.
First, or, ‘in the first place’ (Sranan Tongo), “speaking primarily” (Phillips).
Beware of the leaven of the Pharisees, which is hypocrisy. The metaphor indicates here an all-pervading evil influence. This function may have to be made explicit, e.g. ‘the leaven of the Pharisees, that is (or, I mean), their all-pervading hypocrisy,’ or as a simile and with further shifts, ‘the hypocrisy of the Pharisees which pervades all things like leaven (does).’ The utterance does not warn the disciples against tricks of the hypocritical Pharisees but against their becoming hypocrites themselves. Hence beware of may better be rendered by ‘have nothing to do with’ (Shona 1966), ‘watch-out-for’ (Tae’), ‘keep away from,’ ‘do not become involved in,’ or, again with further shifts, ‘do not become such hypocrites as the Pharisees; their hypocrisy is like an all-pervading leaven.’ For leaven, or, ‘yeast,’ in Wantoat one has to substitute ‘salt.’ For hypocrisy see on 6.42.
Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.