Certainly not a big reduction (4,4)
I believe the answer is:
much less
'a big reduction' is the definition.
The definition suggests a singular noun which matches the answer.
'certainly not' is the wordplay.
'certainly' becomes 'much' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'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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