Seconds: small quantity without a vegetable (7)
I believe the answer is:
spinach
'vegetable' is the definition.
(spinach is a kind of vegetable)
'seconds small quantity without a' is the wordplay.
'seconds small' becomes 's' ('small' suggests an abbreviation).
'quantity' becomes 'pinch' (pinch is a kind of quantity).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'pinch' placed around 'a' is 'pinach'.
's'+'pinach'='SPINACH'
(Other definitions for spinach that I've seen before include "Vegetable plant" , "food" , "Vegetable, source of iron" , "Leafy vegetable" , "Popeye the Sailor's vegetable of choice" .)