First wife and knight on terms (4)
I believe the answer is:
even
'on terms' is the definition.
'first wife and knight' is the wordplay.
'first wife' becomes 'eve' (I've seen this in another clue).
'and' means one lot of letters go next to another.
'knight' becomes 'N' (chess abbreviation).
'eve'+'n'='EVEN'
(Other definitions for even that I've seen before include "Level, calm" , "50-50" , "Nearly" , "Yet; smooth" , "Quits - flat" .)