Snapped, having only forty winks (3)

I believe the answer is:

nap

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'forty winks' is the definition.
(synonyms)

'snapped having only' is the wordplay.
'NAP' is hidden within the letters.
But, I am not sure how this is indicated.

Can you help me to learn more?

(Other definitions for nap that I've seen before include "Quick sleep; raised pile" , "Short sleep on soft surface texture?" , "Card game - quick sleep" , "Kip" , "Pile of material" .)

I've seen this clue in The Mirror.

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