Win over knight buried in secret (7)
I believe the answer is:
convert
'win over' is the definition.
(I know that win over can be written as convert)
'knight buried in secret' is the wordplay.
'knight' becomes 'n' (chess abbreviation).
'buried in' is an insertion indicator (inserted letters get buried).
'secret' becomes 'covert' (similar in meaning).
'n' placed into 'covert' is 'CONVERT'.
(Other definitions for convert that I've seen before include "Proselyte" , "Turn" , "Transform" , "Change from one thing to another" , "Alter for new purpose" .)