Formula the more complicated (7)
I believe the answer is:
theorem
'formula' is the definition.
(I've seen this before)
'the more complicated' is the wordplay.
'complicated' is an anagram indicator (a complex or confusing ordering of the letters).
'more' is an anagram of 'orem'.
'the'+'orem'='THEOREM'
(Other definitions for theorem that I've seen before include "mathematical achievement" , "Proved mathematical hypothesis" , "Proven proposition" , "Formula to be proved" , "Demonstrable proposition" .)