Split up clocks, perhaps – they don’t always work (4-6)

I believe the answer is:

part-timers

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'they don't always work' is the definition.
The definition suggests an adverb but the answer is not.

'split up clocks perhaps' is the wordplay.
'split up' becomes 'part' (part can mean to split or separate).
'clocks perhaps' becomes 'timers'.
'part'+'timers'='PART-TIMERS'

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I've seen this clue in The Guardian.

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