Unable to meet the challenge in bed, turn over one time (3,2,2,2)

I believe the answer is:

not up to it

'unable to meet the challenge' is the definition.
(not up to the challenge)

'in bed turn over one time' is the wordplay.
'in bed' becomes 'not up' (not awake yet).
'turn over' becomes 'to' (t.o. can mean 'turn over' on a written document).
'one' becomes 'i' (Roman numeral).
'time' becomes 't' (abbreviation).
'notup'+'to'+'i'+'t'='NOT UP TO IT'

(Another definition for not up to it that I've seen is " Ill-prepared".)

I've seen this clue in The Guardian.

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