Mark crosses over in a repartee (8)
I believe the answer is:
badinage
'repartee' is the definition.
(badinage is a kind of repartee)
'mark crosses over in a' is the wordplay.
'mark' becomes 'badge' (badging is a kind of marking).
'crosses' means one lot of letters goes inside another.
'over' is a charade indicator (letters next to each other).
'in'+'a' is 'ina'.
'badge' enclosing 'ina' is 'BADINAGE'.
(Other definitions for badinage that I've seen before include "Playful banter" , "Amiable banter" , "Backchat, banter" , "Chaff" , "Ribbing" .)