It bores me in the back (6)
I believe the answer is:
reamer
'it bores' is the definition.
The definition suggests a singular noun which matches the answer.
'me in the back' is the wordplay.
'in' means one lot of letters goes inside another.
'the back' becomes 'rear' (something's rear is its back).
'me' inserted within 'rear' is 'REAMER'.
Can you help me to learn more?
(Other definitions for reamer that I've seen before include "Tool for enlarging drilled holes" , "Dentist's drill for enlarging root canals" , "Boring tool" , "Tool for widening drilled holes" , "Dental tool" .)