Sheep must guard river vessels (5)
I believe the answer is:
ewers
'vessels' is the definition.
(ewer is a kind of vessel)
'sheep must guard river' is the wordplay.
'sheep' becomes 'ewes' (a ewe is a female sheep).
'must guard' is an insertion indicator (inserted letters are held or guarded).
'river' becomes 'r' (abbreviation).
'ewes' going around 'r' is 'EWERS'.
(Other definitions for ewers that I've seen before include "Jugs thrown into sewer" , "Water jugs" , "Vessels with handle and spout" , "Pitchers" , "containers" .)