Thorn in the side of hill guide (9)
I believe the answer is:
tormentor
'thorn in the side' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?
'hill guide' is the wordplay.
'hill' becomes 'tor' (tor is a kind of hill).
'guide' becomes 'mentor' (I've seen this before).
'tor'+'mentor'='TORMENTOR'
'of' acts as a link.
Can you help me to learn more?
(Other definitions for tormentor that I've seen before include "One giving great pain or annoyance" , "Crossword setter maybe" , "Person causing" , "One to harass" , "cause of suffering" .)