Reduced crew on a royal vessel (4)

I believe the answer is:

ewer

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'vessel' is the definition.
(ewer is a type of jug)

'reduced crew on a royal' is the wordplay.
'reduced crew on' becomes 'ew' (I am not sure about this - if you are sure you should believe this answer much more).
'a royal' becomes 'ER' (abbreviation for Elizabeth Regina).
'ew'+'er'='EWER'

Can you help me to learn more?

(Other definitions for ewer that I've seen before include "Large, wide-mouthed jug" , "water-carrier" , "Water jug with wide mouth" , "Container" , "Water-jar" .)

I've seen this clue in the Evening Standard.

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