Ruthless ego, if terribly smug (4-9)
I believe the answer is:
self-righteous
'smug' is the definition.
(I've seen this before)
'ruthless ego if terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'ruthless'+'ego'+'if'='ruthlessegoif'
'ruthlessegoif' with letters rearranged gives 'SELF-RIGHTEOUS'.
(Other definitions for self-righteous that I've seen before include "Convinced that one is morally superior" , "Smugly moralistic" , "Smug; figures he lost (anag.)" , "Unready English monarch" .)