Feel silly around swimmers (4)
I believe the answer is:
eels
'swimmers' is the definition.
(eel is a kind of aquatic vertebrate)
'feel silly around' is the wordplay.
'around' indicates a hidden word.
'EELS' is hidden within 'feel silly'.
(Other definitions for eels that I've seen before include "eg Conger" , "Slender fish" , "They swim" , "Fish (that may be jellied)" , "Odd fish found in the lees" .)