To Rex, trusted adviser is a bully (9)
I believe the answer is:
tormentor
'a bully' is the definition.
(I've seen this before)
'to rex trusted adviser' is the wordplay.
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'trusted adviser' becomes 'mentor' (I've seen this before).
'to'+'r'+'mentor'='TORMENTOR'
'is' is the link.
(Other definitions for tormentor that I've seen before include "Bully" , "Person causing" , "Crossword setter maybe" , "cause of suffering" , "One causes distress" .)