Defender about to overplay ball breaking leg (8)
I believe the answer is:
champion
'defender' is the definition.
(I know that champion is a type of defender)
'about to overplay ball breaking leg' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'to overplay' becomes 'ham' ('ham' can be a synonym of 'overplay').
'ball breaking' becomes 'pi' (I am not sure about this - if you are sure you should believe this answer much more).
'leg' becomes 'on' (cricket term, fielding positions).
'c'+'ham'+'pi'+'on'='CHAMPION'
(Other definitions for champion that I've seen before include "Winner of competition or supporter of cause" , "Gold medallist" , "Back" , "Finest" , "Winner; defender" .)