Distinctive theory is mathematician’s first (3)
I believe the answer is:
ism
'distinctive theory' is the definition.
(I've seen this before)
'is mathematician's first' is the wordplay.
'mathematician's first' becomes 'm' (1st letter of 'mathematician').
'is'+'m'='ISM'
(Other definitions for ism that I've seen before include "form of belief?" , "Belief system" , "Political ideology" , "Informal doctrine - any one" , "Informal doctrine found in Bismarck" .)