Sewer's storage facility regularly tests unit (4)
I believe the answer is:
etui
'sewer's storage facility' is the definition.
(box for holding sewing equipment)
'regularly tests unit' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'tests'+'unit'='testsunit'
The alternate letters of 'testsunit' are 'ETUI'.
(Other definitions for etui that I've seen before include "Small needle case e" , "Ornamental ladies' bag for small articles" , "Small ornamental needle-case" , "Tiny ladies' bag" , "Small case for sewing accessories" .)