Finished thing off before morning (9)

I believe the answer is:

overnight

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'morning' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?

'finished thing off' is the wordplay.
'finished' becomes 'over' (over can mean finished or ended).
'off' indicates anagramming the letters.
'thing' anagrammed gives 'night'.
'over'+'night'='OVERNIGHT'

'before' acts as a link.

Can you help me to learn more?

(Other definitions for overnight that I've seen before include "until tomorrow" , "Extending from dusk to dawn" , "darkness" , "Lasting from dusk to dawn" , "Very quickly" .)

I've seen this clue in The Daily Express.

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