Man cuts to discussing origin of mathematical proposition (7)
I believe the answer is:
theorem
'mathematical proposition' is the definition.
(I've seen this before)
'man cuts to discussing origin' is the wordplay.
'man' becomes 'he' (he refers to a man).
'cuts' means one lot of letters goes inside another (some letters cut their way into other letters).
'discussing origin' becomes 'rem' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'he' put inside 'to' is 'theo'.
'theo'+'rem'='THEOREM'
'of' is the link.
Can you help me to learn more?
(Other definitions for theorem that I've seen before include "this has been established" , "Proof" , "deduction" , "Geometric proposition" , "Proposition not self-evident but proved by a chain of reasoning" .)