Make the sound of a bird (4)
I believe the answer is:
earn
'make' is the definition.
(as in making money)
'sound of a bird' is the wordplay.
'sound of' shows a homophone (sound like).
'a bird' becomes 'erne' (erne is a type of bird).
'erne' is a homophone of 'EARN'.
'the' is the link.
(Other definitions for earn that I've seen before include "Make money" , "Be paid for work" , "Gain by working" , "Gain through labour or work" , "Deserve, gain" .)