Well dressed but useless egghead (4)
I believe the answer is:
dude
'well dressed but useless egghead' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'useless egghead' is the wordplay.
'useless' becomes 'dud' (dud can mean faulty or failing).
'egghead' becomes 'e' (head letter of 'egg').
'dud'+'e'='DUDE'
Can you help me to learn more?
(Other definitions for dude that I've seen before include "An American dandy out west" , "Dandy - city dweller holidaying on ranch" , "Man (American slang)" , "Stylish and confidant man" , "He's no cowboy" .)