Mythical vessel is back, crossing river (4)
I believe the answer is:
argo
'mythical vessel' is the definition.
(I've seen this in another clue)
'back crossing river' is the wordplay.
'back' becomes 'ago' ('years back' means 'years ago').
'crossing' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'ago' going around 'r' is 'ARGO'.
'is' acts as a 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" , "Mythological ship" , "Ancient ship" , "Fabulous ship" .)