Persistent fool is keeping couple unsettled at first (9)

I believe the answer is:

assiduous

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'persistent' is the definition.
(synonyms)

'fool is keeping couple unsettled at first' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'keeping' indicates putting letters inside (some letters kept inside).
'couple' becomes 'duo' (two people).
'unsettled at first' becomes 'u' (I can't explain this - if you can you should give a lot more credence to this answer).
'duo'+'u'='duou'
'is' going around 'duou' is 'iduous'.
'ass'+'iduous'='ASSIDUOUS'

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(Other definitions for assiduous that I've seen before include "Hard-working" , "Persevering" , "Marked by care and persistent effort" , "Showing great care and perseverance" , "Diligent" .)

I've seen this clue in The Times.

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