Line of guards to prevent access (6)
I believe the answer is:
cordon
'line of guards to prevent access' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cordon that I've seen before include "Line of guard to prevent access" , "In cooking, . . . . . . bleu is excellent" , "Riband" , "Chain (of police)" , "Ring of, say, police enclosing something" .)