Subtraction of money? It’s what we infer (9)
I believe the answer is:
deduction
'subtraction' is the definition.
(synonyms)
'of money? it's what we infer' is the wordplay.
I cannot really see how this works, but
'of' could be 'o' and 'o' is found in the answer.
'money?' could be 'tin' (old-fashioned slang term for money) and 'tin' is found within the leftover letters.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for deduction that I've seen before include "Removal; finding" , "Conclusion" , "What's lost" , "Taking off" , "gathering" .)