Upset rubbish instructor annoying person (9)
I believe the answer is:
tormentor
'annoying person' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'upset rubbish instructor' is the wordplay.
'upset' says the letters should be written backwards.
'rubbish' becomes 'rot' (both can mean nonsense).
'instructor' becomes 'mentor'.
'rot' in reverse letter order is 'tor'.
'tor'+'mentor'='TORMENTOR'
Can you help me to learn more?
(Other definitions for tormentor that I've seen before include "cause of pain" , "One causes pain" , "One causes distress" , "Crossword setter maybe" , "Bully" .)