Swelled head (3)
I believe the answer is:
ego
'swelled head' 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 "Conceit - the Latin "I"" , "Sense of self, conceit" , "One's sense of self-importance" , "See 21 Down" , "One's self image" .)