Ancient war galley (7)
I believe the answer is:
trireme
'ancient war galley' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for trireme that I've seen before include "Ancient rowed ship" , "Ancient vessel with three banks of oars on each side" , "Ancient galley or warship" , "Oar-driven battleship of classical times" , "Long rowing boat" .)