Paint — moderate source of art? (7)
I believe the answer is:
tempera
'paint' is the definition.
(type of paint)
'moderate source of art?' is the wordplay.
'moderate' becomes 'temper' (synonyms).
'source of art?' becomes 'a' (I can't justify this - if you can you should believe this answer much more).
'temper'+'a'='TEMPERA'
(Other definitions for tempera that I've seen before include "Method of drawing" , "Artist's medium" , "A paint and a painting technique" , "Means of painting - pet mare (anag)" , "Emulsion used by Renaissance painters" .)