Old warship with two sets of propellers (6)
I believe the answer is:
bireme
'old warship' is the definition.
'bireme' can be an answer for 'warship' (I've seen this before). I'm not sure about the 'old' bit.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for bireme that I've seen before include "with two tiers" , "old vessel?" , "Ancient galley with two banks of oars on each side" , "Type of ancient galley" , "Ancient vessel" .)