Proposition presenting some problem, mathematically (5)
I believe the answer is:
lemma
'proposition' is the definition.
(lemma is a kind of proposition)
'some problem mathematically' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'LEMMA' can be found hidden inside 'problem mathematically'.
'presenting' acts as a link.
(Other definitions for lemma that I've seen before include "heading" , "Subject's division" , "Intermediate proof" , "Subsidiary theorem" , "Proposition" .)