Marched round vault (4)
I believe the answer is:
arch
'vault' is the definition.
(I've seen this before)
'marched round' is the wordplay.
'ARCH' is hidden in amongst the letters.
But, I'm uncertain how this is indicated.
Can you help me to learn more?
(Other definitions for arch that I've seen before include "This could be Wellington" , "Chief - opening with curved top" , "Inner side of foot" , "Roguish, cunning" , "Masonry construction for spanning an opening" .)