Drink up when caught inside vault (6)
I believe the answer is:
cupola
'vault' is the definition.
(I know that cupola is a type of vault)
'drink up when caught' is the wordplay.
'drink' becomes 'cup' (a cup of drink).
'up' is a reversal indicator (in down clue: letters go upwards).
'when caught' becomes 'alo' (I can't justify this - if you can you should believe this answer much more).
'alo' written backwards gives 'ola'.
'cup'+'ola'='CUPOLA'
'inside' acts as a link.
Can you help me to learn more?
(Other definitions for cupola that I've seen before include "St Peter's roof" , "Dome - furnace" , "fireplace" , "Round domed roof" , "Domed ceiling" .)