A line on wall-painting out of doors (8)
I believe the answer is:
alfresco
'out of doors' is the definition.
('alfresco' can be a synonym of 'out of doors')
'a line on wall-painting' is the wordplay.
'line' becomes 'l'.
'on' says to put letters next to each other.
'wall painting' becomes 'fresco' (fresco is a kind of wall painting).
'a'+'l'+'fresco'='ALFRESCO'
(Other definitions for alfresco that I've seen before include "Meal eaten in the outdoors" , "Outdoors, meal perhaps" , "Open-air" , "Out of doors" , "out of the house" .)