Abstract, changing to heretical (11)
I believe the answer is:
theoretical
'abstract' is the definition.
('theoretical' can be a synonym of 'abstract')
'changing to heretical' is the wordplay.
'changing' indicates anagramming the letters.
'to'+'heretical'='toheretical'
'toheretical' with letters rearranged gives 'THEORETICAL'.
(Other definitions for theoretical that I've seen before include "Hypothetical" , "Cheat toiler (anag) - speculative" , "concerned with supposition" , "Academic" , "Not linked to practice" .)