Effect of one medium agreement (6)
I believe the answer is:
impact
'effect' is the definition.
(impact is a kind of effect)
'one medium agreement' is the wordplay.
'one' becomes 'i' (Roman numeral).
'medium' becomes 'm' (as in UK clothing sizes).
'agreement' becomes 'pact' (pact is a kind of agreement).
'i'+'m'+'pact'='IMPACT'
'of' acts as a link.
(Other definitions for impact that I've seen before include "The force fo violent strike" , "Forceful contact between two objects" , "hit, in a big way" , "Result" , "The striking of one body against another" .)