Once in a blue moon, old master goes wrong, turning out bad art (6)
I believe the answer is:
seldom
'once in a blue moon' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
(Other definitions for seldom that I've seen before include "Not 21" , "Not often, infrequently" , "Only occasionally" , "Infrequent" , "Models (anag)" .)