Swimmer in a strict routine (5)
I believe the answer is:
trout
'swimmer' is the definition.
(trout is a kind of aquatic vertebrate)
'in a strict routine' is the wordplay.
'in' indicates the answer is hidden within the clue.
'TROUT' is hidden within 'strict routine'.
(Other definitions for trout that I've seen before include "Tasty fish, perhaps of rainbow kind" , "Schubert Quintet?" , "Schubert work; fish" , "It rises to fly" , "Performance by five players" .)