No good thinking too much of oneself (4)
I believe the answer is:
vain
'no' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for vain that I've seen before include "Conceited, puffed-up" , "Forever looking in the mirror" , "Having too much pride in one's appearance" , "Conceited or unsuccessful" , "Stuck-up? It's hopeless" .)