Sheep approaching river vessel (4)
I believe the answer is:
ewer
'vessel' is the definition.
(ewer is a kind of vessel)
'sheep approaching river' is the wordplay.
'sheep' becomes 'ewe' (I've seen this before).
'approaching' says to put letters next to each other.
'river' becomes 'r'.
'ewe'+'r'='EWER'
(Other definitions for ewer that I've seen before include "Open vessel with handle and spout" , "Big jug" , "Jar for water" , "water-carrier" , "A wide-lipped jug" .)