Swimmer’s new tutor (5)
I believe the answer is:
'swimmer's' is the definition.
(trout is a kind of fish)
'new tutor' is the wordplay.
'new' indicates anagramming the letters.
'tutor' anagrammed gives 'TROUT'.
(Other definitions for trout that I've seen before include "one of a noted quintet" , "air" , "Game fish related to salmon" , "Tutor (anag.)" , "Freshwater game fish" .)