Military blockade (5)
I believe the answer is:
siege
'military blockade' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for siege that I've seen before include "Prolonged attack on a fortified place" , "Military operation cutting off access" , "Surrounding and blockading of town by military" , "Offence in which fortified place is surrounded" , "Persistent attempt to gain" .)