Part of revolutionary rubric is absolutely fundamental (5)
I believe the answer is:
basic
'fundamental' is the definition.
(similar in meaning)
'part of revolutionary rubric is absolutely' is the wordplay.
'revolutionary' says the letters should be written backwards (letters revolve around).
'part of' indicates the answer is hidden within the clue.
'cisab' can be found hidden inside 'rubric is absolutely'.
'cisab' back-to-front is 'BASIC'.
(Other definitions for basic that I've seen before include "Elementary - without extras" , "Radical" , "Staple; ordinary" , "Fundamental or rudimentary" , "At the lowest level" .)