Call of ostrich, primarily avian sound (3)
I believe the answer is:
coo
'avian sound' is the definition.
(I've seen this before)
'call of ostrich primarily' is the wordplay.
'primarily' suggests taking the first letters.
The initial letters of 'call of ostrich' is 'COO'.
(Other definitions for coo that I've seen before include "crumbs" , "Breathe" , "well I never!" , "Sound like a dove" , "Speak fondly" .)