Effect of one treaty on millions (6)
I believe the answer is:
impact
'effect' is the definition.
(impact is a kind of effect)
'one treaty on millions' is the wordplay.
'one' becomes 'i' (Roman numeral).
'treaty' becomes 'pact' ('pact' can be a synonym of 'treaty').
'on' means one lot of letters go next to another.
'millions' becomes 'm' (abbreviation).
'pact' after 'm' is 'mpact'.
'i'+'mpact'='IMPACT'
'of' is the link.
(Other definitions for impact that I've seen before include "Repercussions" , "Strong effect" , "Force or shock of a collision" , "The striking of one thing against another" , "Marked effect or influence" .)