No good thinking too much of oneself (4)

I believe the answer is:

vain

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'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.

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(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" .)

I've seen this clue in The Globe and Mail and The Irish News.

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