Rule about large barrier (4)
I believe the answer is:
wall
'barrier' is the definition.
(a wall acts as a barrier)
'rule about large' is the wordplay.
'rule' becomes 'law' (law is a kind of rule).
'about' is a reversal indicator.
'large' becomes 'L' (eg in clothes sizes).
'law' in reverse letter order is 'wal'.
'wal'+'l'='WALL'
(Other definitions for wall that I've seen before include "Dividing or protecting structure" , "A challenge for the mountaineer" , "Brick construction" , "Side of building/room" , "Brick or stone barrier" .)