Restriction; boundary (5)
I believe the answer is:
limit
I believe this clue is a double definition.
'restriction' is the first definition.
(I've seen this before)
'boundary' is the second definition.
(I know that boundary can be written as limit)
(Other definitions for limit that I've seen before include "Boundary or restriction" , "maximum number" , "Keep in" , "border" , "Extreme point" .)