Large old ship that's broad and slow (5)
I believe the answer is:
largo
'broad and slow' is the definition.
(I've seen this before)
'large old ship' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'old ship' becomes 'argo' (I've seen this in other clues).
'l'+'argo'='LARGO'
'that's' acts as a link.
(Other definitions for largo that I've seen before include "Slow and dignified tempo in music" , "Musically in slow tempo and dignified style" , "Goldfinger part" , "Slow passage" , "Deliberately" .)