Burn key vegetable (5)
I believe the answer is:
chard
'vegetable' is the definition.
(I know that chard is a type of vegetable)
'burn key' is the wordplay.
'burn' becomes 'char' (charring is a kind of burning).
'key' becomes 'd' (musical key).
'char'+'d'='CHARD'
(Other definitions for chard that I've seen before include "Leafy green" , "Variety of beet with edible white leaf stalks" , "Seakale beet" , "Beet with edible leaves and stalks" , "Kind of (Swiss?) beet with edible leaves" .)