Initially feared acolyte knew everything was a sham (4)
I believe the answer is:
fake
'a sham' is the definition.
(I know that fake is a more specific form of the action sham)
'initially feared acolyte knew everything' is the wordplay.
'initially' says to take the initial letters.
The first letters of 'feared acolyte knew everything' is 'FAKE'.
'was' acts as a link.
(Other definitions for fake that I've seen before include "Only pretend" , "N ot genuine" , "False, sham" , "Phoney (like news?)" , "Fraud" .)