Ancient Roman ship (7)
I believe the answer is:
trireme
'ancient roman ship' is the definition.
'trireme' can be an answer for 'ship' (I've seen this before). I also know that both 'ancient' and 'roman' relate to this answer.
This is all the clue.
(Other definitions for trireme that I've seen before include "Ancient Greek or Roman war galley" , "Ancient galley with three tiers of oars on each side" , "Ancient rowed ship" , "Roman/Greek galley" , "Ancient vessel with three banks of oars on each side" .)