Initially Sphinx is explaining very enigmatic riddle (5)

I believe the answer is:

sieve

'riddle' is the definition.
(I've seen this before)

'initially sphinx is explaining very enigmatic' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'sphinx is explaining very enigmatic' is 'SIEVE'.

(Other definitions for sieve that I've seen before include "Mesh used for straining or sifting" , "Fine strainer" , "Screen, sift, strain" , "particle separator" , "Riddle, strain" .)

I've seen this clue in The Telegraph.

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