Certainly not a huge reduction (4,4)

I believe the answer is:

much less

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'a huge reduction' is the definition.
The definition suggests a singular noun which matches the answer.

'certainly not' is the wordplay.
'certainly' becomes 'much' (I can't justify 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'

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I've seen this clue in the Evening Standard.

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