Essential current books by big revolutionary (8)
I believe the answer is:
integral
'essential' is the definition.
(I've seen this before)
'current books by big revolutionary' is the wordplay.
'current' becomes 'I' (symbol for electric current).
'books' becomes 'nt' (New Testament).
'by' says to put letters next to each other.
'big' becomes 'large' (synonyms).
'revolutionary' shows that the letters should be reversed in order.
'large' back-to-front is 'egral'.
'i'+'nt'+'egral'='INTEGRAL'
(Other definitions for integral that I've seen before include "'Inherent, inbuilt (8)'" , "Maths function; whole" , "Altering (anag)" , "whole thing" , "Triangle being essential" .)