Clear over crime, backing court activity (6)
I believe the answer is:
tennis
'court activity' is the definition.
'tennis' can be an answer for 'activity' (tennis is a kind of activity). I also know that 'court' relates to this answer.
'clear over crime backing' is the wordplay.
'clear' becomes 'net' (as in net profit).
'over' says the letters should be written backwards.
'crime' becomes 'sin'.
'backing' shows that the letters should be reversed in order.
'net' in reverse letter order is 'ten'.
'sin' back-to-front is 'nis'.
'ten'+'nis'='TENNIS'
(Other definitions for tennis that I've seen before include "Racket-and-ball game" , "Ball-game" , "Sent in strangely to play the game" , "Game for two or four" , "Racquet sport" .)