A wife in addition 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 addition' is the wordplay.
'wife' becomes 'w' (abbreviation).
'in' means one lot of letters goes inside another.
'addition' becomes 'gain' (I've seen this in another clue).
'a'+'w'='aw'
'aw' placed within 'gain' is 'GAWAIN'.
(Other definitions for gawain that I've seen before include "One of King Arthur's knights" , "K" , "Arthurian knight of the Round Table" , "Man at round table" , "round a table for him and his comrades?" .)