Greatly reduced? Definitely not! (4,4)

I believe the answer is:

much less

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'greatly reduced?' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives can occasionally define each other.

'definitely not' is the wordplay.
'definitely' becomes 'much' (I can't explain this - if you can you should believe this answer much more).
'not' becomes 'less' (I've seen this in other clues).
'much'+'less'='MUCH LESS'

Can you help me to learn more?

I've seen this clue in the Evening Standard.

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