The more complicated mathematical proof (7)
I believe the answer is:
theorem
'mathematical proof' is the definition.
'theorem' can be an answer for 'proof' (I've seen this before). I am not sure about the 'mathematical' bit.
'the more complicated' is the wordplay.
'complicated' is an anagram indicator (a complex or confusing ordering of the letters).
'more' anagrammed gives 'orem'.
'the'+'orem'='THEOREM'
Can you help me to learn more?
(Other definitions for theorem that I've seen before include "mathematical achievement" , "speculation" , "Mathematical rule" , "Proven proposition" , "Formula to be proved" .)