Irregular theme or proposition (7)
I believe the answer is:
theorem
'proposition' is the definition.
(I know that theorem is a type of proposition)
'irregular theme or' is the wordplay.
'irregular' indicates anagramming the letters.
'theme'+'or'='themeor'
'themeor' anagrammed gives 'THEOREM'.
(Other definitions for theorem that I've seen before include "(Maths) proposition" , "Statement to be proved in maths" , "Proposition not self-evident but proved by a chain of reasoning" , "Proved mathematical hypothesis" , "Proven proposition" .)