Artistic subject in muddle at intervals (4)
I believe the answer is:
nude
'artistic subject' is the definition.
(subject painted in the nude)
'in muddle at intervals' is the wordplay.
'at intervals' means one should take alternating letters (letters are taken at regular intervals).
'in'+'muddle'='inmuddle'
The alternating letters of 'inmuddle' are 'NUDE'.
(Other definitions for nude that I've seen before include "Naked art subject" , "lacking cover" , "Without cover" , "Naked, bare" , "Wearing no clothes" .)