Horn carried over border (4)
I believe the answer is:
abut
'border' is the definition.
(synonyms)
'horn carried over' is the wordplay.
'horn' becomes 'tuba' (instrument in the horn section).
'carried over' says the letters should be written in reverse.
'tuba' backwards is 'ABUT'.
(Other definitions for abut that I've seen before include "Meet" , "Adjoin -- 19 (rev)" , "Touch upon" , "Neighbour" , "Be next door to" .)