New missile submarine, say, moving home? (10)
I believe the answer is:
'moving home?' is the definition.
(type of residence which moves down canals)
'new missile submarine say' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'missile' becomes 'arrow' (arrow is a kind of missile).
'submarine say' becomes 'boat' (submarine is a type of boat).