Millions! Everyone in shopping centre! (4)
I believe the answer is:
mall
'shopping centre' is the definition.
(a mall is a large shopping complex)
'millions everyone' is the wordplay.
'millions' becomes 'm' (abbreviation).
'everyone' becomes 'all' (all people).
'm'+'all'='MALL'
'in' is the link.
(Other definitions for mall that I've seen before include "Retail complex" , "Shopping area closed to vehicles" , "Large enclosed shopping area" , "Sheltered walk or promenade" , "Traffic-free street" .)