Unhurried passage of old vessel on lake (5)
I believe the answer is:
largo
'unhurried passage' is the definition.
(musical passage to be played slowly)
'old vessel on lake' is the wordplay.
'old vessel' becomes 'argo' (I've seen this before).
'on' means one lot of letters go next to another.
'lake' becomes 'L' (geographical abbreviation).
'argo' after 'l' is 'LARGO'.
'of' acts as a link.
(Other definitions for largo that I've seen before include "Moderately slowly" , "Slow movement in music" , "Mus. slow movement" , "In a slow tempo (mus.)" , "Played with dignified slowness" .)