Swimmer curtailed children's game (7)
I believe the answer is:
sardine
'swimmer' is the definition.
(sardine is a kind of aquatic vertebrate)
'curtailed children's game' is the wordplay.
'curtailed' means to remove the last letter.
'children's game' becomes 'sardines'.
'sardines' with its final letter taken off is 'SARDINE'.
(Other definitions for sardine that I've seen before include "Swimmer" , "Small fish, usually canned" , "Fish in tin?" , "(Cramped?) fish" , "Fish (often tinned)" .)