Mate by employing skill with knight (7)
I believe the answer is:
partner
'mate' is the definition.
(I've seen this before)
'by employing skill with knight' is the wordplay.
'by' becomes 'per' (**).
'employing' means one lot of letters goes inside another.
'skill' becomes 'art' (I've seen this before).
'with' means one lot of letters go next to another.
'knight' becomes 'n' (chess).
'art'+'n'='artn'
'per' enclosing 'artn' is 'PARTNER'.
(Other definitions for partner that I've seen before include "south to north, perhaps" , "Associate, in business or private life" , "Other half?" , "Mate or business associate" , "Comrade" .)