Aquatic birds reportedly go south (5)
I believe the answer is:
terns
'aquatic birds' is the definition.
(tern is a kind of aquatic bird)
'reportedly go south' is the wordplay.
'reportedly' shows a homophone (sound like).
'go' becomes 'turn' (as in one's go or turn in a game).
'south' becomes 's' (abbreviation).
'turn' is a homophone of 'tern'.
'tern'+'s'='TERNS'
(Other definitions for terns that I've seen before include "Sea bird with long forked tail" , "Seabirds" , "Sea birds with long forked tails" , "Rents out these birds" , "Aquatic birds" .)