Touch instrument that's reversed (4)
I believe the answer is:
abut
'touch' is the definition.
(abutting is a kind of touching)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for abut that I've seen before include "End or lean against, say a building" , "Adjoin, touch" , "Impinge" , "Adjoin at one end" , "Be next to -- share a boundary" .)