Takes skin test regularly (4)
I believe the answer is:
sits
'takes' is the definition.
(sit can mean to take an exam)
'skin test regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'skin'+'test'='skintest'
The alternate letters of 'skintest' are 'SITS'.
(Other definitions for sits that I've seen before include "Takes a seat for the painter" , "Perches" , "Poses for a painter" , "Poses for portrait" , "Takes a chair" .)