Crack swimmer claiming victory (5)
I believe the answer is:
solve
'crack' is the definition.
(thesaurus)
'swimmer claiming victory' is the wordplay.
'swimmer' becomes 'sole' (sole is a kind of aquatic vertebrate**).
'claiming' means one lot of letters goes inside another.
'victory' becomes 'v'.
'sole' going around 'v' is 'SOLVE'.
(Other definitions for solve that I've seen before include "Unravel (a clue)" , "Voles (anag.)" , "Find an answer to (a problem)" , "Complete crossword" , "Find the solution to puzzle or problem" .)