First Lady and knight on terms (4)
I believe the answer is:
even
'on terms' is the definition.
'first lady and knight' is the wordplay.
'first lady' becomes 'eve' (first woman in the Bible).
'and' is a charade indicator (letters next to each other).
'knight' becomes 'N' (chess abbreviation).
'eve'+'n'='EVEN'
(Other definitions for even that I've seen before include "of consistent quality" , "Level as odds may be" , "Regular; calm" , "Flush" , "Steady or smooth" .)