Border on the back of a large brass instrument (4)
I believe the answer is:
abut
'border on' is the definition.
(I've seen this in another clue)
'back of a large brass instrument' is the wordplay.
'back of' shows that the letters should be reversed in order (I've seen 'back' mean this).
'a large brass instrument' becomes 'tuba' (I've seen this before).
'tuba' reversed gives 'ABUT'.
'the' is the link.
(Other definitions for abut that I've seen before include "Adjoin, border" , "Neighbour" , "Be next to -- share a boundary" , "Adjoin, press up against" , "Cease (from activity)" .)