Her tome revealed proposition (7)
I believe the answer is:
theorem
'proposition' is the definition.
(theorem is a kind of proposition)
'her tome revealed' is the wordplay.
'revealed' indicates an anagram.
'her'+'tome'='hertome'
'hertome' is an anagram of 'THEOREM'.
(Other definitions for theorem that I've seen before include "The more it changed to a proposition to be proved" , "Statement to be proved in maths" , "speculation" , "Deduced formula" , "mathematical achievement" .)