Moderating playtime instruction often led by ma (3,6)

I believe the answer is:

non troppo

'moderating playtime instruction' is the definition.
(musical instruction to play music moderately)

I cannot understand how the remainder of the clue works.

(Other definitions for non troppo that I've seen before include "not over for players" , "Not too much (in music)" .)

I've seen this clue in The Times.

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