Variantly: "the venom of snakes is their wine": Your "moderate" ones, the fearers of sin among you — their venom will be as that of snakes. "and the cruel gall ('rosh') of asps": Your heads (roshim) will be like cruel asps (to you).
R. Nechemiah expounded it as referring to the nations, viz.: Most certainly you are of the wine of Sodom and of the planting of Amorah, disciples of the primal serpent, who deceived Adam and Eve.
"Bitter clusters are theirs": Your (the nations') great ones — their fear hangs over them (the Jews) as (the venom of) a snake. ("eshkol" connotes "greatness," as in (Michah 7:1-2) "There is no eshkol to eat, etc."