I'm standing by agreement with striking effect (6)
I believe the answer is:
impact
'striking effect' is the definition.
'impact' can be an answer for 'effect' (impact is a kind of effect). I am unsure of the 'striking' bit.
'i'm standing by agreement' is the wordplay.
'standing by' is a charade indicator (letters next to each other).
'agreement' becomes 'pact' (a pact is an agreement or deal).
'im'+'pact'='IMPACT'
'with' acts as a link.
Can you help me to learn more?
(Other definitions for impact that I've seen before include "Forceful consequence" , "Repercussion" , "Force of colllision or blow" , "Forcible contact between two things" , "Force of blow or collision" .)