Son, no longer effective, determined to quit (5,3)

I believe the answer is:

sworn off

'determined to quit' is the definition.
(to swear something off is to quit it)

'son no longer effective' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'no longer effective' becomes 'worn off' (something that's worn off is no longer effective).
's'+'wornoff'='SWORN OFF'

I've seen this clue in The Times.

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