Beginning to chop firm vegetable (5)
I believe the answer is:
chard
'vegetable' is the definition.
(chard is a kind of vegetable)
'beginning to chop firm' is the wordplay.
'beginning to' says to take the initial letters.
'firm' becomes 'hard' ('hard' can be similar in meaning to 'firm').
The first letter of 'chop' is 'c'.
'c'+'hard'='CHARD'
(Other definitions for chard that I've seen before include "Green vegetable" , "Type of beet" , "vegetable crop" , "Leaf stalks from a kind of beet" , "Variety of beet with edible leaves" .)