Mythical ship, on lake, proceeding slowly (5)
I believe the answer is:
largo
'slowly' is the definition.
(I've seen this in another clue)
'mythical ship on lake' is the wordplay.
'mythical ship' becomes 'argo' (ship in Greek myth).
'on' is a charade indicator (letters next to each other).
'lake' becomes 'L' (geographical abbreviation).
'argo' after 'l' is 'LARGO'.
'proceeding' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for largo that I've seen before include "(Musically) slow in tempo and broad in manner" , "To be played slowly and broadly" , "Mus. slow movement" , "Played with dignified slowness" , "Moderately slowly" .)