Consequences of one initially minor agreement (6)
I believe the answer is:
impact
'consequences' is the definition.
(thesaurus)
'one initially minor agreement' is the wordplay.
'one' becomes 'i' (Roman numeral).
'initially' indicates taking the first letters.
'agreement' becomes 'pact' (pact is a kind of agreement).
The initial letter of 'minor' is 'm'.
'i'+'m'+'pact'='IMPACT'
'of' acts as a link.
(Other definitions for impact that I've seen before include "The striking of one thing against another" , "The striking of one body against another" , "Forceful striking" , "Forceful consequence" , "Forceful contact between two objects" .)