Border upon (4)
I believe the answer is:
abut
'border upon' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for abut that I've seen before include "Adjoin, press up against" , "Cease (from activity)" , "Tuba (anag)" , "Of building or land, end or lean against" , "Adjoin at one end" .)