Old ship’s slow movement leaving loch (4)
I believe the answer is:
argo
'old ship's' is the definition.
(I've seen this in other clues)
'slow movement leaving loch' is the wordplay.
'slow movement' becomes 'largo' (I've seen this before).
'leaving' suggests deleting specific letters.
'loch' becomes 'l'.
'largo' with 'l' taken out is 'ARGO'.
(Other definitions for argo that I've seen before include "Old ship" , "Ship involved in the quest for the Golden Fleece" , "Agro about the constellation" , "Mythical ship" , "Stars once recognised in the south" .)