Border control (5)
I believe the answer is:
limit
I believe this clue is a double definition.
'border' is the first definition.
(I've seen this before)
'control' is the second definition.
(limiting is a kind of controlling)
(Other definitions for limit that I've seen before include "Curtail" , "Extreme point" , "Ceiling" , "Confine within bounds" , "Constraint" .)