Old ship is slowly heading off (4)
I believe the answer is:
argo
'old ship' is the definition.
(I've seen this before)
'slowly heading off' is the wordplay.
'slowly' becomes 'largo' (I've seen this before).
'heading off' means to remove the first letter.
'largo' with its initial letter removed is 'ARGO'.
'is' is the link.
(Other definitions for argo that I've seen before include "Ship in which Jason searched for the Golden Fleece" , "Transport to find the Golden Fleece" , "Old ship" , "Old sailing vessel" , "Jason's vessel" .)