A Liberal painting in the open air (8)
I believe the answer is:
alfresco
'in the open air' is the definition.
(I've seen this before)
'a liberal painting' is the wordplay.
'a liberal' becomes 'al' (I've seen this in other clues).
'painting' becomes 'fresco' (fresco is a kind of painting).
'al'+'fresco'='ALFRESCO'
(Other definitions for alfresco that I've seen before include "In the open air" , "Outside" , "Meal eaten in the outdoors" , "Open-air" , "Outdoors, meal perhaps" .)