Busy insect beginning to taste vegetable (4)
I believe the answer is:
beet
'vegetable' is the definition.
(beet is a kind of vegetable)
'busy insect beginning to taste' is the wordplay.
'busy insect' becomes 'bee' (I've seen this before).
'beginning to' says to take the initial letters.
The first letter of 'taste' is 't'.
'bee'+'t'='BEET'
(Other definitions for beet that I've seen before include "Root vegetable producing sugar" , "Root used as a source of sugar" , "Genus of plant with succulent root" , "Root, maybe source of sugar" , "Crop plant, sugar source" .)