Swimmer enters freely (3)
I believe the answer is:
eel
'swimmer' is the definition.
(eel is a kind of fish)
'enters freely' is the wordplay.
'enters' indicates a hidden word (I've seen 'entering' mean this).
'EEL' is hidden in the letters of 'freely'.
(Other definitions for eel that I've seen before include "Conger, e.g." , "Slippery person" , "Proverbially slippery fish" , "Slippery figure" , "User of electric organ" .)