Many an old ship is slow in movement (5)
I believe the answer is:
largo
'slow in movement' is the definition.
(musical term)
'many an old ship' is the wordplay.
'many' becomes 'l' ().
'an old ship' becomes 'argo' (I've seen this before).
'l'+'argo'='LARGO'
'is' acts as a link.
(Other definitions for largo that I've seen before include "(Of music) broad and slow - Bond villain" , "In a slow tempo (mus.)" , "at stately speed" , "moving slowly" , "Moderately slowly" .)