Viking vessel (8)
'viking vessel' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for longship that I've seen before include "Viking vessel" , "Viking craft" , "Viking boat" .)
(Other definitions for longboat that I've seen before include "Viking ship" , "one attacking from the north" , "Large vessel launched from a sailing ship" , "one getting a row?" , "Vessel used by the Vikings" .)