Border on being a tuba composition (4)
I believe the answer is:
abut
'border on' is the definition.
'a tuba composition' is the wordplay.
'composition' is an anagram indicator.
'tuba' is an anagram of 'ABUT'.
'being' is the link.
(Other definitions for abut that I've seen before include "Join" , "Touch, lean on" , "Lie next to" , "Cease (from activity)" , "Touch upon" .)