Non-government warship commissioned to plunder the enemy (9)
I believe the answer is:
privateer
(Other definitions for privateer that I've seen before include "Non-government owned warship" , "Pare rivet down for a ship authorised to make war" , "Non-naval vessel of old, licensed to attack" , "Warship operating under a letter of marque" , "Government-commissioned vessel authorised to prey on enemy shipping" .)