Metal pick uncovered a turnip, possibly (8)
I believe the answer is:
brassica
'turnip possibly' is the definition.
'metal pick uncovered a' is the wordplay.
'metal' becomes 'brass' (brass is a kind of metal).
'uncovered' indicates the central letters.
The centre of 'pick' is 'ic'.
'brass'+'ic'+'a'='BRASSICA'
(Other definitions for brassica that I've seen before include "Cabbage, broccoli, etc." , "Swede, say" , "Vegetable" , "Cabbages, broccoli etc." , "Cabbage, say" .)