One in on scam set free? He can’t get off (9)

I believe the answer is:

insomniac

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'free? he can't get off' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

'one in on scam set' is the wordplay.
'one' becomes 'i' (Roman numeral).
'set' indicates an anagram (instruction to set the letters).
'in'+'on'+'scam'='inonscam'
'inonscam' is an anagram of 'nsomniac'.
'i'+'nsomniac'='INSOMNIAC'

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(Other definitions for insomniac that I've seen before include "One who can't sleep" , "One unable to sleep" , "Can't drop off" , "This person can't sleep" , "Habitually sleepless person" .)

I've seen this clue in The Guardian.

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