Painting regularly turned, we suspect (4)
I believe the answer is:
Here is my best explanation:
'painting' is the definition.
(nude is a kind of painting)
'regularly turned we suspect' is the subsidiary indication.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'suspect' indicates an anagram.
The alternating letters of 'turnedwe' are 'unde'.
'unde' is an anagram of 'nude'.
(Other definitions for nude that I've seen before include "In the buff", "Classic art subject", "example of erotic art", "totally exposed", "Having nothing on", "Bare", "Naked person", "Undraped figure", "lacking cover" and "Wearing one's birthday suit".)