What sewer may carry tests unit regularly (4)
I believe the answer is:
etui
'what sewer may carry' is the definition.
I can't judge whether this definition defines the answer.
'tests unit regularly' 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'.
Can you help me to learn more?
(Other definitions for etui that I've seen before include "Small case for small articles" , "Small sewing bag" , "Small French case" , "box" , "Small ornamental needle-case" .)