Swimmer changed tutor (5)
I believe the answer is:
trout
'swimmer' is the definition.
(trout is a kind of aquatic vertebrate)
'changed tutor' is the wordplay.
'changed' is an anagram indicator.
'tutor' with letters rearranged gives 'TROUT'.
(Other definitions for trout that I've seen before include "Performance by five players" , "tickled to death?" , "Edible fish" , "Fish of salmon family" , "Oily fish" .)