Problem over Japanese sport (4)
I believe the answer is:
sumo
'japanese sport' is the definition.
'problem over' is the wordplay.
'problem' becomes 'sum' (arithmetic problem).
'over' becomes 'o' (cricket abbreviation).
'sum'+'o'='SUMO'
(Other definitions for sumo that I've seen before include "Grappling" , "Style of traditional Japanese wrestling" , "slappers fighting?" , "foreign form of fighting" , "Kind of wrestling" .)