It once involved two banks in a row! (6)
I believe the answer is:
bireme
'it once involved two banks in a row' is the definition.
The definition suggests a singular noun which matches the answer.
This is the entire clue.
(Other definitions for bireme that I've seen before include "ancient craft" , "Old craft with two banks of oars" , "Type of ancient galley" , "Ancient rowed warship" , "Ancient vessel" .)