Al with this painting would be in the open air (6)
I believe the answer is:
fresco
'al with this painting would be in the open air' is the definition.
'fresco' can be an answer for 'painting' (fresco is a kind of painting). I'm not sure about the rest of the definition.
This is the entire clue.
(Other definitions for fresco that I've seen before include "Artwork" , "Wall-painting" , "Mode of wall painting" , "Painting using watercolours on plaster" , "Watercolour done on wet plaster" .)