Over-hasty artist — say no more! (4)
I believe the answer is:
rash
'over-hasty' is the definition.
(I've seen this before)
'artist say no more' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'say no more' becomes 'sh' ('sh' means stop talking).
'ra'+'sh'='RASH'
(Other definitions for rash that I've seen before include "Ill-considered - outbreak" , "Incautious" , "Impulsive, having skin ailment" , "Precipitate" , "Flood" .)