Starts to dispute Oslo staging European superheavyweight bouts (5)
I believe the answer is:
doses
'bouts' is the definition.
(dose can mean a bout of illness)
'starts to dispute oslo staging european superheavyweight' is the wordplay.
'starts to' says to take the initial letters.
The initial letters of 'dispute oslo staging european superheavyweight' is 'DOSES'.
(Other definitions for doses that I've seen before include "Measured amounts of medicine" , "Measured portions of medicine" , "Prescribed quantities of medicine" , "draughts" , "Potion portions" .)