Bigheadedness (3)
I believe the answer is:
ego
'bigheadedness' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ego that I've seen before include "Personal pride" , "One's self image" , "(Person's) sense of self-esteem" , "In Virgil I" , "Self-esteem, conceit" .)