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