Crossword Genius

Old warship with two sets of propellers (6)

Ross

I believe the answer is:

bireme

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in The Globe and Mail.
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