Vegetable drinks knocked back by a companion (7)
I believe the answer is:
spinach
'vegetable' is the definition.
(spinach is a kind of vegetable)
'drinks knocked back by a companion' is the wordplay.
'drinks' becomes 'nips' (miniature drink bottles**).
'knocked back' says the letters should be written in reverse.
'by' says to put letters next to each other.
'companion' becomes 'ch' (I've seen this in other clues).
'nips' reversed gives 'spin'.
'spin'+'a'+'ch'='SPINACH'
(Other definitions for spinach that I've seen before include "something to eat" , "Blackboard stand" , "Leaf vegetable" , "Popeye's strength-giver" , "Vegetable plant" .)