Swimmer taking no part, maybe (6)
I believe the answer is:
tarpon
'swimmer' is the definition.
(tarpon is a kind of aquatic vertebrate)
'no part maybe' is the wordplay.
'maybe' is an anagram indicator.
'no'+'part'='nopart'
'nopart' anagrammed gives 'TARPON'.
'taking' acts as a link.
(Other definitions for tarpon that I've seen before include "Great fish of Florida" , "Large silvery game fish of coastal waters off Florida" , "No part for this large fish" .)