Greek bird making beastly noise (5)
I believe the answer is:
growl
'noise' is the definition.
(I know that growl is a noise)
'greek bird' is the wordplay.
'greek' becomes 'gr' (abbreviation for Greek).
'bird' becomes 'owl' (owl is a kind of bird).
'gr'+'owl'='GROWL'
'making beastly' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for growl that I've seen before include "Make an aggressive sound" , "Emit low rumbling sounds" , "Noise of an angry dog" , "warn of attack" , "Low rumble" .)