Theory exists with maths, at first (3)
I believe the answer is:
ism
'theory' is the definition.
(I have seen 'Distinctive theory' mean 'ism' so perhaps 'theory' could also mean 'ism')
'exists with maths at first' is the wordplay.
'exists' becomes 'is' ('be' can be a synonym of 'exist').
'with' means one lot of letters go next to another.
'at first' says to take the initial letters.
The first letter of 'maths' is 'm'.
'is'+'m'='ISM'
(Other definitions for ism that I've seen before include "Informal noun or suffix for doctrine or practice" , "Suffix denoting doctrine or practise" , "Informal doctrine - any one" , "Belief system" , "Any ideological doctrine" .)