Certainly not one of the big spenders (5)
I believe the answer is:
miser
This could be a double definition.
'certainly not one' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'of the big spenders' is the second definition.
The definition suggests a singular noun which matches the answer.
Can you help me to learn more?
(Other definitions for miser that I've seen before include "Stingy hoarder" , "Avaricious hoarder" , "screw" , "Tightwad, hoarder" , "Mean person" .)