Notes gulf developing a certain amount (6)

I believe the answer is:

bagful

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'a certain amount' is the definition.
'bagful' can be an answer for 'amount' (bagful is a kind of amount). I am unsure of the 'a certain' bit.

'notes gulf developing' is the wordplay.
'notes' becomes 'ba'.
'developing' indicates anagramming the letters (letters developed into a new form).
'gulf' anagrammed gives 'gful'.
'ba'+'gful'='BAGFUL'

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I've seen this clue in The Mirror.

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