Tutor trained swimmer (5)
I believe the answer is:
trout
'swimmer' is the definition.
(trout is a kind of aquatic vertebrate)
'tutor trained' is the wordplay.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'tutor' anagrammed gives 'TROUT'.
(Other definitions for trout that I've seen before include "Swimmer" , "Commonly caught fish" , "Rainbow, for example" , "Freshwater game fish" , "one often tickled?" .)