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 can't explain this - if you can you should give a lot more credence to this answer).
'involving' is an insertion indicator.
'navy' becomes 'RN' (abbreviation for Royal Navy).
'us' placed around '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" .)