Prepare for war — notice a fleet prepared to attack our shores (6)
I believe the answer is:
armada
'fleet prepared to attack our shores' is the definition.
(fleet of war ships)
'prepare for war notice a' is the wordplay.
'prepare for war' becomes 'arm' (as in arming with weapons).
'notice' becomes 'ad' (both can mean an advertisement).
'arm'+'ad'+'a'='ARMADA'
(Other definitions for armada that I've seen before include "Large Spanish fleet" , "Spanish fleet sent in 1588 to escort an invasion of England" , "group of ships" , "Fleet Drake defeated" , "1588 sea battle" .)