Giant bird sitting on a nest's top (5)
I believe the answer is:
titan
'giant' is the definition.
('titan' can be a synonym of 'giant')
'bird sitting on a nest's top' is the wordplay.
'bird' becomes 'tit' (I've seen this before**).
'sitting on' is a charade indicator (letters next to each other).
'top' suggests taking the first letters.
The first letter of 'nests' is 'n'.
'tit'+'a'+'n'='TITAN'
(Other definitions for titan that I've seen before include "colossal one" , "A son of Uranus and Gaea" , "Person of great importance" , "Giant -- rocket" , "Old Greek god or very strong person" .)