Surround, as a castle (7)
I believe the answer is:
besiege
'surround as a castle' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for besiege that I've seen before include "Blockade" , "Surround (a place) with hostile intent" , "Surround (town etc) with armed forces" , "'Harass, beleaguer (7)'" , "Surround (a stronghold)" .)