Ancient ship on lake moving slowly (5)
I believe the answer is:
largo
'moving slowly' is the definition.
(musical term)
'ancient ship on lake' is the wordplay.
'ancient ship' becomes 'argo' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'lake' becomes 'L' (geographical abbreviation).
'argo' put after 'l' is 'LARGO'.
(Other definitions for largo that I've seen before include "taking it steady" , "Played with dignified slowness" , "Slow and dignified, like Handel's composition" , "Broad and slow (mus.)" , "In a slow tempo (mus.)" .)