One in mood smashed up vault (5,4)
I believe the answer is:
onion dome
'up vault' is the definition.
The definition suggests a singular noun which matches the answer.
'one in mood smashed' is the wordplay.
'smashed' is an anagram indicator.
'one'+'in'+'mood'='oneinmood'
'oneinmood' with letters rearranged gives 'ONION DOME'.
Can you help me to learn more?
(Other definitions for onion dome that I've seen before include "Byzantine structure?" , "Roof feature such as on Brighton Pavilion" , "part of Russian church?" , "Roof shaped like a pungent vegetable, on a Russian church, perhaps" , "Architectural feature" .)