Effect of setter's agreement (6)
I believe the answer is:
impact
'effect' is the definition.
(I know that impact is a type of effect)
'setter's agreement' is the wordplay.
'setter's' becomes 'I'm' (I've seen this in other clues).
'agreement' becomes 'pact' (a pact is an agreement or deal).
'im'+'pact'='IMPACT'
'of' is the link.
(Other definitions for impact that I've seen before include "The force of one body against another" , "Strong effect" , "Force exerted by collision" , "Forceful contact between two objects" , "Forcible contact; marked influence" .)