Relaxed, not on watch (3,5)

I believe the answer is:

off guard

'relaxed' is the definition.
(thesaurus)

'not on watch' is the wordplay.
'not on' becomes 'off' ('off' is the opposite to 'on').
'watch' becomes 'guard' (guarding is a kind of watching).
'off'+'guard'='OFF-GUARD'

(Another definition for off guard that I've seen is " Unprepared".)

I've seen this clue in The Telegraph.

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