Certain number were troubled working (6,2)

I believe the answer is:

nailed on

'certain' is the definition.
(both can mean fixed or definite)

'number were troubled working' is the wordplay.
'number' becomes 'n' (n can represent any number).
'were troubled' becomes 'ailed' (ail can mean to be troubled).
'working' becomes 'on' (on can mean working or activated).
'n'+'ailed'+'on'='NAILED ON'

(Another definition for nailed on that I've seen is " horse so favoured?".)

I've seen this clue in The Guardian.

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