Devil-worshipper transforms as saint beginning to tremble (8)
I believe the answer is:
satanist
'devil-worshipper' is the definition.
(I've seen this before)
'transforms as saint beginning to tremble' is the wordplay.
'transforms' is an anagram indicator (I've seen this in other clues).
'beginning to' indicates taking the first letters.
The first letter of 'tremble' is 't'.
'as'+'saint'='assaint'
'assaint' is an anagram of 'satanis'.
'satanis'+'t'='SATANIST'
(Other definitions for satanist that I've seen before include "Saint sat with devil worshipper" , "Devil devotee" , "A follower of the devil" , "Devil-worshipper" , "diabolical fellow" .)