Reason, reportedly, for bird calls (4)
I believe the answer is:
caws
'bird calls' is the definition.
(I've seen this in another clue)
'reason reportedly' is the wordplay.
'reason' becomes 'cause' (I've seen this in another clue).
'reportedly' shows a homophone (sound like).
'cause' sounds like 'CAWS'.
'for' acts as a link.
(Other definitions for caws that I've seen before include "Rook calls" , "Cries of murder" , "Harsh cries of crows" .)