Regularly test music case (4)
I believe the answer is:
etui
'case' is the definition.
(case for needlework equipment)
'regularly test music' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'test'+'music'='testmusic'
The alternate letters of 'testmusic' are 'ETUI'.
(Other definitions for etui that I've seen before include "Small case for small things" , "Small ornamental case" , "Ornamental bag for needles etc." , "The Case of the Seamstress" , "Small sewing bag" .)