Search for swimmer (4)
I believe the answer is:
fish
I believe this clue is a double definition.
'search' is the first definition.
(fishing is a kind of searching)
'swimmer' is the second definition.
(fish is a kind of aquatic vertebrate)
'for' is the link.
(Other definitions for fish that I've seen before include "To try to catch" , "Aquatic creature(s)" , "Burbot or turbot, for example" , "Search" , "counter in various games" .)