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' is a reversal indicator.
'tuba' back-to-front is '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" .)