Vessel right behind sheep (4)
I believe the answer is:
ewer
'vessel' is the definition.
(ewer is a type of jug)
'right behind sheep' is the wordplay.
'right' becomes 'r' (common abbreviation).
'behind' says to put letters next to each other.
'sheep' becomes 'ewe' (a ewe is a female sheep).
'r' after 'ewe' is 'EWER'.
(Other definitions for ewer that I've seen before include "29 [VESSEL]" , "Sort of jug" , "Large, wide-mouthed jug" , "Jar for water" , "A type of jug" .)