Raise army to protect island (5)
I believe the answer is:
hoist
'raise' is the definition.
(hoisting is a kind of raising)
'army to protect island' is the wordplay.
'army' becomes 'host' (I've seen this before).
'to protect' indicates putting letters inside.
'island' becomes 'i'.
'host' enclosing 'i' is 'HOIST'.
(Other definitions for hoist that I've seen before include "elevator" , "Lift using pulley and ropes" , "I shot (anag)" , "Elevate" , "Raise or haul up, say flag" .)