Vessel always filled with water, primarily (4)
I believe the answer is:
ewer
'vessel' is the definition.
(I know that ewer is a type of vessel)
'always filled with water primarily' is the wordplay.
'always' becomes 'eer' (poetic way of saying ever).
'filled with' means one lot of letters goes inside another.
'primarily' says to take the initial letters.
The first letter of 'water' is 'w'.
'eer' placed around 'w' is 'EWER'.
(Other definitions for ewer that I've seen before include "A type of jug" , "Pitcher, water jug" , "Decorative pitcher" , "Sort of jug" , "A wide-mouthed water-jug" .)