Some unfortunate swimmer (4)
I believe the answer is:
tuna
'swimmer' is the definition.
(tuna is a kind of aquatic vertebrate)
'some unfortunate' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'TUNA' is hidden in the letters of 'unfortunate'.
(Other definitions for tuna that I've seen before include "Large fish found in a nut" , "Common food fish" , "Fish; edible cactus" , "Large fish for the strange aunt" , "Yellow-finned fish" .)