Brief exercise involving Navy vessels (4)
I believe the answer is:
urns
'vessels' is the definition.
(urn is a kind of vessel)
'brief exercise involving navy' is the wordplay.
'brief exercise' becomes 'us' (I am not sure about this - if you are sure you should believe this answer much more).
'involving' is an insertion indicator.
'navy' becomes 'RN' (abbreviation for Royal Navy).
'us' enclosing 'rn' is 'URNS'.
Can you help me to learn more?
(Other definitions for urns that I've seen before include "Containers for ashes" , "Runs to get large vases" , "Tall vases" , "Vase-shaped vessels" , "Runs to get funeral vases" .)