Start to cook tough vegetable (5)
I believe the answer is:
chard
'vegetable' is the definition.
(I know that chard is a type of vegetable)
'start to cook tough' is the wordplay.
'start to' says to take the initial letters.
'tough' becomes 'hard' (similar in meaning).
The initial letter of 'cook' is 'c'.
'c'+'hard'='CHARD'
(Other definitions for chard that I've seen before include "salad" , "vegetable crop" , "Type of beet" , "Kind of (Swiss?) beet with edible leaves" , "Variety of beet with edible white leaf stalks" .)