Professor, a rotter, chopped up mice (8)
I believe the answer is:
academic
'professor' is the definition.
(I know that academic is a type of academic)
'a rotter chopped up mice' is the wordplay.
'rotter' becomes 'cad' (I've seen this before).
'chopped up' indicates an anagram.
'mice' anagrammed gives 'emic'.
'a'+'cad'+'emic'='ACADEMIC'
(Other definitions for academic that I've seen before include "This person may be a professor" , "Scholarly as opposed to practical (8)" , "Unimportant" , "Theoretical or scholarly - often not practical" , "Scholar - of no practical relevance" .)