Store's retired madame checking juniors regularly work (8)
I believe the answer is:
emporium
'store's' is the definition.
(I've seen this before)
'retired madame checking juniors regularly work' is the wordplay.
'retired' says the letters should be written in reverse.
'madame' becomes 'mme' (abbreviation).
'checking' indicates putting letters inside (check can mean to control or arrest).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'work' becomes 'op' (abbreviation for opus).
The alternate letters of 'juniors' are 'uir'.
'uir'+'op'='uirop'
'mme' placed around 'uirop' is 'muiropme'.
'muiropme' reversed gives 'EMPORIUM'.
(Other definitions for emporium that I've seen before include "A large imposing shop" , "A large shop or centre of commerce" , "Large and ostentatious retail establishment" , "Big shop" , "Make up memoir about a very large shop" .)