Other plays with me, one's proven (7)
I believe the answer is:
theorem
'one's proven' is the definition.
I can't judge whether this defines the answer.
'other plays with me' is the wordplay.
'plays with' indicates anagramming the letters.
'other' with its letters mixed up with 'me' makes 'THEOREM'.
Can you help me to learn more?
(Other definitions for theorem that I've seen before include "Proof" , "Hero met (anag) - principle that's proven" , "The more it changed to a proposition to be proved" , "Mathematical formula" , "Algebraic rule" .)