Simple painting, smaller (7)
I believe the answer is:
artless
'simple' is the definition.
('artless' can be similar in meaning to 'simple')
'painting smaller' is the wordplay.
'painting' becomes 'art' (I've seen this before).
'smaller' becomes 'less'.
'art'+'less'='ARTLESS'
(Other definitions for artless that I've seen before include "Without pretension" , "Natural like slaters" , "Without guile or affectation" , "Free from cunning" , "Simple, unaffected" .)