Preserve old pot in felt backing (5,3)

I believe the answer is:

extra jam

'preserve' is the definition.

'old pot in felt backing' is the wordplay.
'old' becomes 'ex'.
'pot' becomes 'jar'.
'in' is an insertion indicator.
'felt' becomes 'mat'.
'backing' says the letters should be written backwards.
'jar' inserted into 'mat' is 'majart'.
'majart' reversed gives 'trajam'.
'ex'+'trajam'='EXTRA JAM'

(Another definition for extra jam that I've seen is " Preserve following regulation".)

I've seen this clue in The Times.

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