Narcissist, say, is wearing too much (7)
I believe the answer is:
egotist
'narcissist' is the definition.
(thesaurus)
'say is wearing too much' is the wordplay.
'say' becomes 'eg'.
'wearing' is an insertion indicator.
'too much' becomes 'ott' (short for 'over the top' - informal British).
'is' placed within 'ott' is 'otist'.
'eg'+'otist'='EGOTIST'
(Other definitions for egotist that I've seen before include "Conceited and self-centred person" , "Self-obsessed person" , "One who talks only of self" , "Bighead" , "A self-absorbed and conceited person" .)