Swelled head? Say nothing! (3)
I believe the answer is:
ego
'swelled head?' is the definition.
(I've seen this in another clue)
'say nothing' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'nothing' becomes 'O' (looks like zero - 0).
'eg'+'o'='EGO'
(Other definitions for ego that I've seen before include "Part of the mind" , "One's sense of self-importance" , "Morale" , "(Person's) sense of self-esteem" , "(Self-assertive) personality" .)