Energy-filled paintings found around 505 AD (6)
I believe the answer is:
advert
'ad' is the definition.
(synonyms)
'energy-filled paintings found around 505' is the wordplay.
'energy' becomes 'e' (physics symbol as in E=mcv).
'filled' means one lot of letters goes inside another.
'paintings' becomes 'art' (painting is a type of artwork).
'found around' is an insertion indicator.
'505' becomes 'dv' (I can't explain this - if you can you should give a lot more credence to this answer).
'e' going within 'art' is 'aert'.
'aert' enclosing 'dv' is 'ADVERT'.
Can you help me to learn more?
(Other definitions for advert that I've seen before include "Plug (abbr)" , "Promotional notice" , "Publicity announcement" , "Refer to the red vat" , "Publicity item" .)