Vegetable: usual bargain (7)
I believe the answer is:
parsnip
'vegetable' is the definition.
(parsnip is a kind of vegetable)
'usual bargain' is the wordplay.
'usual' becomes 'par'.
'bargain' becomes 'snip' (slang term for a good deal).
'par'+'snip'='PARSNIP'
(Other definitions for parsnip that I've seen before include "A white root vegetable" , "Yellow root vegetable" , "Pips ran oddly through the vegetable" , "Pale carrot-like vegetable" , "food" .)