Small collar around a vegetable (7)
I believe the answer is:
spinach
'vegetable' is the definition.
(spinach is a kind of vegetable)
'small collar around a' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'collar' becomes 'pinch' (synonyms).
'around' means one lot of letters goes inside another.
'pinch' placed around 'a' is 'pinach'.
's'+'pinach'='SPINACH'
(Other definitions for spinach that I've seen before include "Blackboard stand" , "leaves on plate?" , "Some greens" , "food" , "Popeye's strength-giver" .)