Ancient mariners snore terribly (5)
I believe the answer is:
norse
'ancient mariners' is the definition.
(I've seen this in another clue)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for norse that I've seen before include "These men were Vikings" , "Relating to ancient Scandinavian" , "Of old Scandinavia" , "Northern European" , "Language - senor! (anag)" .)