Cause's partner (6)
I believe the answer is:
effect
'cause's partner' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for effect that I've seen before include "Consequence caused by something else" , "Consequence of some previous happening" , "Resultant change" , "See 7" , "Change produced by impact or other cause" .)