It's not clear that they no longer have the ability to concentrate (3,2,5)

I believe the answer is:

out of focus

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'it's not clear that they no longer have' is the definition.
The definition suggests an adverb but the answer is not.

'ability to concentrate' is the wordplay.
'ability' becomes 'outof' (I can't explain this - if you can you should give a lot more credence to this answer).
'to concentrate' becomes 'focus' (synonyms).
'outof'+'focus'='OUT OF FOCUS'

'the' acts as a link.

Can you help me to learn more?

I've seen this clue in The Irish Times.

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