Mythical ship sinks below lake slowly (5)
I believe the answer is:
largo
'slowly' is the definition.
(I've seen this in another clue)
'mythical ship sinks below lake' is the wordplay.
'mythical ship' becomes 'argo' (ship in Greek myth).
'sinks below' means one lot of letters go next to another.
'lake' becomes 'L' (geographical abbreviation).
'argo' put after 'l' is 'LARGO'.
(Other definitions for largo that I've seen before include "Slow movement in music" , "Mus. slow movement" , "Musical piece" , "Deliberately" , "Played with dignified slowness" .)