Business mass market leaders entered in force (8)
I believe the answer is:
commerce
'business' is the definition.
(thesaurus)
'mass market leaders entered in force' is the wordplay.
'leaders' suggests taking the first letters.
'entered in' indicates putting letters inside.
'force' becomes 'coerce' ('coerce' can be a synonym of 'force'**).
The initial letters of 'mass market' is 'mm'.
'mm' put within 'coerce' is 'COMMERCE'.
(Other definitions for commerce that I've seen before include "Buying/selling activity" , "Trading exchanges" , "The buying and selling of commodities and services" , "Trading activity" , "Traffic" .)