Slow, large vessel (5)
I believe the answer is:
largo
'slow' is the definition.
(musical term)
'large vessel' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'vessel' becomes 'argo' (ship in Greek myth).
'l'+'argo'='LARGO'
(Other definitions for largo that I've seen before include "Slow passage" , "Moderately slowly" , "Musical piece" , "Broad and slow (mus.)" , "Deliberately" .)