Giant bird -- a nuisance at first (5)
I believe the answer is:
titan
'giant' is the definition.
('titan' can be a synonym of 'giant')
'bird a nuisance at first' is the wordplay.
'bird' becomes 'tit' (I've seen this before**).
'at first' indicates taking the first letters.
The first letter of 'nuisance' is 'n'.
'tit'+'a'+'n'='TITAN'
(Other definitions for titan that I've seen before include "Very strong or important person" , "One huge" , "Atlas, possibly" , "Person of heroic stature" , "Person of great size" .)