A South American bird to foster, we hear (4)
I believe the answer is:
rhea
'a south american bird' is the definition.
(I've seen this in another clue)
'foster we hear' is the wordplay.
'foster' becomes 'rear' (both can mean to bring up or raise).
'we hear' shows a homophone (sound like).
'rear' sounds like 'RHEA'.
'to' is the link.
(Other definitions for rhea that I've seen before include "China grass" , "Fast-running flightless bird" , "Ostrich-like bird of South America" , "Large flightless bird" , "Moon" .)