Hem in (7)
I believe the answer is:
besiege
'hem in' is the definition.
(synonyms)
This is all the clue.
(Other definitions for besiege that I've seen before include "Crowd round oppressively" , "Enclose, preventing ingress or egress" , "'Harass, beleaguer (7)'" , "Blockade" , "Surround (a place) with hostile intent" .)