Ancient galley with two banks of oars (6)
I believe the answer is:
bireme
'ancient galley with two banks of oars' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for bireme that I've seen before include "Type of ancient galley" , "Old ship" , "Ancient rowed warship" , "Rowed boat long ago" , "old vessel?" .)