Proposition Tom here? Bizarre! (7)
I believe the answer is:
theorem
'proposition' is the definition.
(theorem is a kind of proposition)
'tom here? bizarre' is the wordplay.
'bizarre' indicates an anagram.
'tom'+'here'='tomhere'
'tomhere' is an anagram of 'THEOREM'.
(Other definitions for theorem that I've seen before include "Algebraic rule" , "Hero met (anag) - principle that's proven" , "(Maths) proposition" , "Mathematical proposition proved by reasoning" , "Proven proposition" .)