Slow movement of mythical ship on lake (5)
I believe the answer is:
largo
'slow movement' is the definition.
(I've seen this before)
'mythical ship on lake' is the wordplay.
'mythical ship' becomes 'argo' (I've seen this before**).
'on' is a charade indicator (letters next to each other).
'lake' becomes 'l'.
'argo' after 'l' is 'LARGO'.
'of' acts as a link.
(Other definitions for largo that I've seen before include "Broad and slow (mus.)" , "Goldfinger part" , "To be played slowly and broadly" , "Slow movement in music" , "(Musically) slow in tempo and broad in manner" .)