When retired, Betty Ford pens a classic song (5)

I believe the answer is:

fytte

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'a classic song' is the definition.
The definition suggests a singular noun which matches the answer.

'when retired betty ford pens' is the wordplay.
'when retired' says the letters should be written in reverse (retire can mean to go back).
'pens' says the answer is hidden in the clue (pen can mean to contain or cage).
'ettyf' can be found hidden inside 'betty ford'.
'ettyf' reversed gives 'FYTTE'.

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I've seen this clue in The Independent.

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