Column holding aloft a rounded vault (6)
I believe the answer is:
cupola
'rounded vault' is the definition.
(architectural term)
'column holding aloft a' is the wordplay.
'column' becomes 'col'.
'holding' indicates putting letters inside.
'aloft' becomes 'up' (up above).
'col' enclosing 'up' is 'cupol'.
'cupol'+'a'='CUPOLA'
(Other definitions for cupola that I've seen before include "St Peter's roof" , "Small dome on a roof" , "fireplace" , "Domed ceiling" , "Rounded dome" .)