School class painting in green messily (12)
I believe the answer is:
kindergarten
'school' is the definition.
(I know that kindergarten is a type of school)
'class painting in green messily' is the wordplay.
'class' becomes 'kind' (I've seen this in another clue).
'painting' becomes 'art' (painting is an art).
'in' is an insertion indicator.
'messily' indicates anagramming the letters.
'green' with letters rearranged gives 'ergen'.
'art' placed inside 'ergen' is 'ergarten'.
'kind'+'ergarten'='KINDERGARTEN'
(Other definitions for kindergarten that I've seen before include "Playgroup" , "Pre-primary school class" , "Nursery school" , "Pre-school institution for little children" , "Infant school" .)