Giant bird, wild, heading off (7)
I believe the answer is:
titanic
'giant' is the definition.
(I've seen this before)
'bird wild heading off' is the wordplay.
'bird' becomes 'tit' (tit is a kind of bird).
'wild' becomes 'manic' (I've seen this before).
'heading off' means to remove the first letter.
'manic' with its first letter removed is 'anic'.
'tit'+'anic'='TITANIC'
(Other definitions for titanic that I've seen before include "Huge - disaster struck this ship" , "Famous doomed transatlantic liner" , "Hugely strong (but lost!)" , "made of metal" , "Ill-fated liner" .)