A large picture painted on a wall outside (8)
I believe the answer is:
alfresco
'outside' is the definition.
(I've seen this before)
'a large picture painted on a wall' is the wordplay.
'large' becomes 'l'.
'picture painted on a wall' becomes 'fresco' (I've seen this before).
'a'+'l'+'fresco'='ALFRESCO'
(Other definitions for alfresco that I've seen before include "Outside" , "Open-air" , "where it's cool" , "Out of doors" , "exposed to the fresh air" .)