Saint to encourage swimmer (8)
I believe the answer is:
sturgeon
'swimmer' is the definition.
(type of fish)
'saint to encourage' is the wordplay.
'saint' becomes 'st' (abbreviation).
'to encourage' becomes 'urgeon' (urging on is a kind of encouraging).
'st'+'urgeon'='STURGEON'
(Other definitions for sturgeon that I've seen before include "Main resident" , "Large fish, valued for its roe" , "The fish for caviars" , "Source of caviar" , "Fish - got nurse (anag)" .)