Swimmer had endless sport on balance (8)
I believe the answer is:
porpoise
'swimmer' is the definition.
(I've seen this before)
'endless sport on balance' is the wordplay.
'endless' means to look at the middle letters.
'on' says to put letters next to each other.
'balance' becomes 'poise' (synonyms).
The centre of 'sport' is 'por'.
'por'+'poise'='PORPOISE'
'had' acts as a link.
(Other definitions for porpoise that I've seen before include "Marine mammal" , "Animal" , "Sea creature (reported close behind by the 10 to the 9!)" , "Member of dolphin family" , "Small whale with blunt snout" .)