Concert for millions (4)
I believe the answer is:
prom
'concert' is the definition.
(abbreviation for promenade concert)
'for millions' is the wordplay.
'for' becomes 'pro' (I've seen this before).
'millions' becomes 'm' (abbreviation).
'pro'+'m'='PROM'
(Other definitions for prom that I've seen before include "US high school dance" , "Henry Wood concert" , "School ball (N America)" , "Albert Hall concert" , "BBC summer concert" .)