I'm getting agreement to have an effect (6)
I believe the answer is:
impact
'an effect' is the definition.
(impact is a kind of effect)
'i'm getting agreement' is the wordplay.
'getting' says to put letters next to each other.
'agreement' becomes 'pact' (a pact is an agreement or deal).
'im'+'pact'='IMPACT'
'to have' acts as a link.
(Other definitions for impact that I've seen before include "Force or shock of a collision" , "Repercussions" , "The striking of one body against another" , "Result" , "Force of blow or collision" .)