Listen! Swimmer's heading off (4)
I believe the answer is:
hark
'listen' is the definition.
(harking is a kind of listening)
'swimmer's heading off' is the wordplay.
'swimmer' becomes 'shark' (shark is a kind of fish).
'heading off' means to remove the first letter.
'shark' with its first letter removed is 'HARK'.
(Other definitions for hark that I've seen before include "Listen as in olden days" , "Listen (exclamation)" , "Hear" .)