A wife in win for knight (6)
I believe the answer is:
gawain
'for knight' is the definition.
(Sir Gawain is a knight in Arthurian legend)
'a wife in win' is the wordplay.
'wife' becomes 'w' (abbreviation).
'in' means one lot of letters goes inside another.
'win' becomes 'gain' (synonyms).
'a'+'w'='aw'
'aw' put within 'gain' is 'GAWAIN'.
(Other definitions for gawain that I've seen before include "round a table for him and his comrades?" , "K" , "One of King Arthur's knights" , "Man at round table" , "Arthurian knight of the Round Table" .)