Off and on, malmsey left one with speech improvement (12)

I believe the answer is:

amelioration

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'improvement' is the definition.
(amelioration is a kind of improvement)

'off and on malmsey left one with speech' is the wordplay.
'off and on' means one should take alternating letters (letters taken on and off, on a regular basis).
'left' becomes 'l' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'with speech' becomes 'oration' (I can't justify this - if you can you should give a lot more credence to this answer).
The alternate letters of 'malmsey' are 'ame'.
'ame'+'l'+'i'+'oration'='AMELIORATION'

Can you help me to learn more?

(Another definition for amelioration that I've seen is " Improvement".)

I've seen this clue in The Times.

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