"Nonsense!" returned the advisor, "He's a pest" (9)
I believe the answer is:
tormentor
'a pest' is the definition.
(thesaurus)
'nonsense returned the advisor he's' is the wordplay.
'nonsense' becomes 'rot' (rot can mean nonsense or rubbish).
'returned' is a reversal indicator.
'the advisor he' becomes 'mentor' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rot' back-to-front is 'tor'.
'tor'+'mentor'='TORMENTOR'
Can you help me to learn more?
(Other definitions for tormentor that I've seen before include "Who would tease" , "One causes distress" , "One causes pain" , "One giving great pain or annoyance" , "Person causing" .)