Swelter uncomfortably before round’s beginning for fighter (8)
I believe the answer is:
wrestler
'fighter' is the definition.
(wrestler is a kind of fighter)
'swelter uncomfortably before round's beginning' is the wordplay.
'uncomfortably' is an anagram indicator.
'before' means one lot of letters go next to another.
'beginning' indicates taking the first letters.
The first letter of 'rounds' is 'r'.
'swelter' anagrammed gives 'wrestle'.
'wrestle'+'r'='WRESTLER'
'for' acts as a link.
(Other definitions for wrestler that I've seen before include "Athlete trying to pin opponent to the ground" , "Professional fighter" , "One fighting in ring" , "Sumo type" , "Grappling fighter" .)