Police line (6)
I believe the answer is:
cordon
'police line' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for cordon that I've seen before include "Line of guards to prevent entry" , "A ring of police or military" , "Police line; trained fruit tree" , "Circle of policemen" , "Line of guards; rope" .)