US state must wash regularly (4)
I believe the answer is:
utah
'us state' is the definition.
(I've seen this before)
'must wash regularly' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'must'+'wash'='mustwash'
The alternating letters of 'mustwash' are 'UTAH'.
(Other definitions for utah that I've seen before include "Mormons are found here in a hut" , "State of last Winter Olympics" , "US state settled by the Mormons" , "Brigham Young state" , "Bryce Canyon US state" .)