Giant bird a non-starter (5)
I believe the answer is:
titan
'giant' is the definition.
(titans are giant mythological creatures)
'bird a non-starter' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'starter' suggests taking the first letters.
The initial letter of 'non' is 'n'.
'tit'+'a'+'n'='TITAN'
(Other definitions for titan that I've seen before include "bigwig" , "A giant; Saturn moon" , "Atlas, say" , "enormous thing" , "very big" .)