Uranium on Royal Navy vessel (3)
I believe the answer is:
urn
'vessel' is the definition.
(I know that urn is a type of pot)
'uranium on royal navy' is the wordplay.
'uranium' becomes 'U' (U is the chemical symbol for uranium).
'on' means one lot of letters go next to another.
'royal navy' becomes 'RN' (abbreviation for Royal Navy).
'u'+'rn'='URN'
(Other definitions for urn that I've seen before include "Holder of the Ashes?" , "Large pot for making tea" , "Ashes trophy" , "Ornamental vessel" , "Funereal vase" .)