Impression made by this writer’s agreement (6)
I believe the answer is:
impact
'impression' is the definition.
(I've seen this before)
'by this writer's agreement' is the wordplay.
'by' says to put letters next to each other.
'this writer's' becomes 'Im' (referring to the person writing the clues).
'agreement' becomes 'pact' (a pact is an agreement or deal).
'im'+'pact' is 'IMPACT'.
'made' acts as a link.
(Other definitions for impact that I've seen before include "Force exerted by collision" , "Result" , "Marked effect or influence" , "Impression" , "Forceful striking" .)