Oblige millions to enter trade (8)
I believe the answer is:
commerce
'trade' is the definition.
(commercial activity)
'oblige millions to enter' is the wordplay.
'oblige' becomes 'coerce' (coercing is a kind of obliging).
'millions' becomes 'mm' (m is an abbreviation for million).
'to enter' is an insertion indicator.
'coerce' enclosing 'mm' is 'COMMERCE'.
(Other definitions for commerce that I've seen before include "The buying and selling of commodities and services" , "Activity of buying and selling" , "Trading activity" , "Business dealings" , "Sales and purchases of goods and services" .)