He's unable to serve any longer (5)

I believe the answer is:

lifer

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'he's unable to serve any longer' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.

This is the entire clue.

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(Other definitions for lifer that I've seen before include "Rifle (anag.)" , "Prisoner serving the maximum term" , "Person serving long sentence" , "One behind bars for ever" , "person given sizeable bird" .)

I've seen this clue in The Globe and Mail.

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