Vessel originally used by navy (3)
I believe the answer is:
urn
'vessel' is the definition.
(urn is a type of container)
'originally used by navy' is the wordplay.
'originally' suggests taking the first letters.
'by' is a charade indicator (letters next to each other).
'navy' becomes 'RN' (abbreviation for Royal Navy).
The first letter of 'used' is 'u'.
'u'+'rn'='URN'
(Other definitions for urn that I've seen before include "Receptacle for ashes of the dead" , "Tall stemmed vase" , "Huge teapot" , "Run to get funeral vase" , "Vaselike vessel on a pedestal" .)