Impetuous artist is endlessly shy (4)
I believe the answer is:
rash
'impetuous' is the definition.
(I know that impetuous can be written as rash)
'artist is endlessly shy' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'is endlessly' means to remove the last letter.
'shy' with its last letter taken off is 'sh'.
'ra'+'sh'='RASH'
(Other definitions for rash that I've seen before include "Impulsive - eruption" , "Daredevil" , "Foolish; skin problem" , "Ill-considered - break-out" , "Skin redness" .)