One's sense of self-importance (3)
I believe the answer is:
ego
'one's sense of self-importance' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ego that I've seen before include "Self-confidence" , "Conscious mind" , "One's self-image" , "Conceit - the Latin "I"" , "Conscious self" .)