Search for a swimmer (4)
I believe the answer is:
fish
I believe this is a double definition.
'search' is the first definition.
(I know that fish is a type of search)
'a swimmer' is the second definition.
(fish is a kind of aquatic vertebrate)
'for' acts as a link.
(Other definitions for fish that I've seen before include "occupant of tank" , "rock, perhaps" , "Aquarium" , "Weatherman" , "Search" .)