Rejected seed is cause of downfall (7)

I believe the answer is:

nemesis

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'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" .)

I've seen this clue in The Guardian.

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