Two swimmers, the first heard complaining (7)
I believe the answer is:
railing
'complaining' is the definition.
(railing is a kind of complaining)
'two swimmers the first' is the wordplay.
'two swimmers' can mean two replacements for 'swimmer'.
'swimmer' becomes 'ling' (ling is a type of fish).
'swimmer' becomes 'rai' (I can't explain this - if you can you should believe this answer much more).
'the first' means one lot of letters go next to another.
'ling' after 'rai' is 'RAILING'.
'heard' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for railing that I've seen before include "complaints procedure?" , "Gin lair (anag.)" , "border of park?" , "Mocking" , "barrier appears" .)