Added once pictures were mounted (5)
I believe the answer is:
extra
'added' is the definition.
(similar in meaning)
'pictures were mounted' is the wordplay.
'pictures' becomes 'art'.
'were' becomes 'xe' (I can't justify this - if you can you should believe this answer much more).
'mounted' says the letters should be written in reverse (in a down clue, letters mounted up).
'art'+'xe'='artxe'
'artxe' back-to-front is 'EXTRA'.
'once' acts as a link.
Can you help me to learn more?
(Other definitions for extra that I've seen before include "eg Wide; additional" , "One has no lines" , "outside" , "Left over" , "Studio crowd player" .)