Big bird, head hidden in fear (7)
I believe the answer is:
titanic
'big' is the definition.
('titanic' can be similar in meaning to 'big')
'bird head hidden in fear' is the wordplay.
'bird' becomes 'tit' (I've seen this before).
'head hidden in' means to remove the first letter.
'fear' becomes 'panic' (panic is a kind of fear).
'panic' with its initial letter taken away is 'anic'.
'tit'+'anic'='TITANIC'
(Other definitions for titanic that I've seen before include "Huge - disaster struck this ship" , "Huge, like doomed liner that struck iceberg" , "Sunken vessel" , "In attic (anag.)" , "Extremely large" .)