A journey across river for legendary vessel (4)
I believe the answer is:
argo
'legendary vessel' is the definition.
(ship in Greek myth)
'a journey across river' is the wordplay.
'journey' becomes 'go' (journey means to go or travel).
'across' is an insertion indicator.
'river' becomes 'r' (abbreviation).
'a'+'go'='ago'
'ago' enclosing 'r' is 'ARGO'.
'for' is the link.
(Other definitions for argo that I've seen before include "Ship involved in the quest for the Golden Fleece" , "Old boat" , "Transport to find the Golden Fleece" , "Stars once recognised in the south" , "Jason's legendary ship" .)