Black and greasy when cooked (6)
I believe the answer is:
boiled
'cooked' is the definition.
(boiling is a kind of cooking)
'black and greasy' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'and' means one lot of letters go next to another.
'greasy' becomes 'oiled' (I've seen this before).
'b'+'oiled'='BOILED'
'when' acts as a link.
(Other definitions for boiled that I've seen before include "Heated to point at which vapour forms" , "Cooked in hot water" .)