Raise an army to surround one (5)
I believe the answer is:
hoist
'raise' is the definition.
(hoisting is a kind of raising)
'an army to surround one' is the wordplay.
'an army' becomes 'host' (host is a kind of army).
'to surround' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'host' enclosing 'i' is 'HOIST'.
(Other definitions for hoist that I've seen before include "Device for lifting, often attached to 5 down" , "Give a lift" , "Raise with ropes" , "set of flags" , "Raise aloft" .)