Surprised having forty winks (6,7)

I believe the answer is:

caught napping

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'surprised' is the definition.
(thesaurus)

'forty winks' is the wordplay.
I cannot quite understand how this works, but
'forty winks' could be 'nap' (synonyms) and 'nap' is located in the answer.
This explanation may well be incorrect...

'having' acts as a link.

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

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