Portly individual starts fight (5,3)
I believe the answer is:
round one
'starts fight' is the definition.
(first round of a boxing match)
'portly individual' is the wordplay.
'portly' becomes 'round' (portly means round or plump).
'individual' becomes 'one' (an individual is one person).
'round'+'one'='ROUND ONE'
(Other definitions for round one that I've seen before include "Start of fight" , "Where armour should be" , "start of contest" .)