Raise army to besiege island (5)
I believe the answer is:
hoist
'raise' is the definition.
(hoisting is a kind of raising)
'army to besiege island' is the wordplay.
'army' becomes 'host' (I've seen this before).
'to besiege' is an insertion indicator.
'island' becomes 'i'.
'host' enclosing 'i' is 'HOIST'.
(Other definitions for hoist that I've seen before include "Substance forming outer skeleton of insects, etc" , "I shot (anag)" , "Jack" , "Raise aloft" , "One that could raise" .)