Feeling about swimmer (3)
I believe the answer is:
eel
'swimmer' is the definition.
(eel is a type of fish)
'feeling about' is the wordplay.
'about' indicates the answer is hidden within the clue (the clue goes around or about the answer).
'EEL' is hidden within 'feeling'.
(Other definitions for eel that I've seen before include "Unagi, in a sushi bar" , "Slippery character" , "Serpentine fish" , "Sea creature like conger" , "Slithery fish" .)