Rejected seed is cause of downfall (7)
I believe the answer is:
nemesis
'cause of downfall' is the definition.
'nemesis' can be an answer for 'downfall' (I've seen this before). I am not sure about the 'cause of' bit.
'rejected seed is' is the wordplay.
'rejected' is a reversal indicator.
'seed' becomes 'semen' ('semen' can be a synonym of 'seed').
'semen' backwards is 'nemes'.
'nemes'+'is'='NEMESIS'
Can you help me to learn more?
(Other definitions for nemesis that I've seen before include "The Greek goddess of divine retribution and vengeance" , "rival who can't be beaten" , "Nee Miss (anag.)" , "(Fate's) retribution" , "Come-uppance" .)