Offence and difficulty mounting in ball game (6)
I believe the answer is:
tennis
'ball game' is the definition.
(I know that tennis is a type of ball game)
'offence and difficulty mounting' is the wordplay.
'offence' becomes 'sin' (a sin is an offence or transgression).
'and' means one lot of letters go next to another.
'difficulty' becomes 'net' (net can mean a difficulty).
'mounting' is a reversal indicator (in a down clue, letters mount or go up).
'sin'+'net'='sinnet'
'sinnet' backwards is 'TENNIS'.
'in' acts as a link.
(Other definitions for tennis that I've seen before include "Sport Dwight Davis gave trophy for" , "Sent in (anag.)" , "Sent in strangely to play the game" , "Henman's game" , "Ball sport" .)