Riddle that is replacing extremely original riddle (5)
I believe the answer is:
sieve
'riddle' is the definition.
(I know that riddle has the is a more specific form of the action sieve)
'is replacing extremely original riddle' is the wordplay.
'replacing' indicates an anagram (re-placing: placing the letters again).
'extremely original riddle' becomes 'eve' (I can't explain this - if you can you should give a lot more credence to this answer).
'is' anagrammed gives 'si'.
'si'+'eve'='SIEVE'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for sieve that I've seen before include "used by the Jumblies" , "Mesh used for straining or sifting" , "Utensil for straining or sifting food" , "Utensil with holes for straining" , "Device for separating lumps" .)