Giant bird heads for Alpine nest (5)
I believe the answer is:
titan
'giant' is the definition.
(I know this)
'bird heads for alpine nest' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'heads for' indicates taking the first letters.
The initial letters of 'alpine nest' is 'an'.
'tit'+'an'='TITAN'
(Other definitions for titan that I've seen before include "Person of heroic stature" , "Atlas, say" , "Goddess" , "Taint in a legendary giant" , "Giant god of Gr. myth" .)