Siesta over, switched on the building in Rome (8)
I believe the answer is:
pantheon
'building in rome' is the definition.
'pantheon' can be an answer for 'building' (I have seen 'Roman building' mean 'pantheon' so perhaps 'building' could also mean 'pantheon'). I am not certain of the 'in rome' bit.
'siesta over switched on the' is the wordplay.
'siesta' becomes 'nap' (I've seen this before**).
'over' says the letters should be written in reverse.
'switched' is an anagram indicator (I've seen 'switch' mean this).
'nap' reversed gives 'pan'.
'on'+'the'='onthe'
'onthe' is an anagram of 'theon'.
'pan'+'theon'='PANTHEON'
Can you help me to learn more?
(Other definitions for pantheon that I've seen before include "church" , "Circular temple in Rome" , "memorial" , "All the gods of a people or religion" , "Ancient Roman temple" .)