Signal swimmer in lead to return (5)
I believe the answer is:
bleep
'signal' is the definition.
(I have seen 'Electronic signal ' mean 'bleep' so perhaps 'signal' could also mean 'bleep')
'swimmer in lead to return' is the wordplay.
'swimmer' becomes 'eel' (eel is a kind of fish).
'in' indicates putting letters inside.
'lead' becomes 'Pb' (Pb is the chemical symbol for lead).
'to return' says the letters should be written backwards.
'eel' going into 'pb' is 'peelb'.
'peelb' written backwards gives 'BLEEP'.
(Other definitions for bleep that I've seen before include "High-pitched signal from device - you're needed" , "warning sound" , "Electronic noise signal" , "Electronic sound (used over profanities)" , "Swearing substitute" .)