Slow lake craft, once (5)
I believe the answer is:
largo
'slow' is the definition.
(musical term)
'lake craft once' is the wordplay.
'lake' becomes 'L' (geographical abbreviation).
'craft once' becomes 'argo' (I have seen 'Old craft' mean 'argo' so perhaps 'craft' could also mean 'argo'. I am not sure about the 'once' bit.).
'l'+'argo'='LARGO'
Can you help me to learn more?
(Other definitions for largo that I've seen before include "Broad and slow (mus.)" , "(Musically) slow in tempo and broad in manner" , "Musically in slow tempo and dignified style" , "Goldfinger part" , "Florida key: musical slow movement" .)