Fast old music speed (5)

I believe the answer is:

lento

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'fast old music speed' is the definition.
Both the definition and answer are adverbs. Maybe there's an association between them I don't understand?

'fast old' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'lent'+'o'='LENTO'

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(Other definitions for lento that I've seen before include "Old sayings" , "Slow musically" , "Scored slowly" , "Musically low speed" , "How one scores slowly" .)

I've seen this clue in The Mirror.

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