Firm working to enclose road barrier (6)
I believe the answer is:
cordon
'barrier' is the definition.
(I've seen this before)
'firm working to enclose road' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'working' becomes 'on' (on can mean working or activated).
'to enclose' is an insertion indicator.
'road' becomes 'rd' (abbreviation).
'co'+'on'='coon'
'coon' going around 'rd' is 'CORDON'.
(Other definitions for cordon that I've seen before include "Line of police preventing passage" , "Ring of people guarding something" , "Circle of police" , "Barrier surrounding an area" , "'Line, of police say, around an area to prevent entry (6)'" .)