Touch horn when reversing (4)
I believe the answer is:
abut
'touch' is the definition.
(abutting is a kind of touching)
'horn when reversing' is the wordplay.
'horn' becomes 'tuba' (instrument in the horn section).
'when reversing' says the letters should be written backwards.
'tuba' written backwards gives 'ABUT'.
(Other definitions for abut that I've seen before include "Adjoin, be contiguous to" , "Adjoin, border" , "Join" , "Lie next to" , "Be next door to" .)