Ancient vessel with three banks of oars on each side (7)
I believe the answer is:
trireme
'ancient vessel with three banks of oars on each side' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for trireme that I've seen before include "Galley with three banks of oars" , "Ancient Greek or Roman war galley" , "Ancient warship with three banks of oars" , "Ancient galley with three tiers of oars on each side" , "Greek warship" .)