The riddle is possibly woman! (5)
I believe the answer is:
Here is my best explanation:
'the riddle' is the definition.
(I've seen this before)
'is possibly woman' is the subsidiary indication.
'possibly' indicates an anagram.
'woman' becomes 'eve' (Eve is an example**).
'is' anagrammed gives 'si'.
(Other definitions for sieve that I've seen before include "Fine strainer", "Sift, strain", "Strain", "Kitchen utensil", "Craft for Jumblies", "Wire mesh utensil", "Strainer", "Forgetful head is like it", "Straining device" and "Utensil with holes for straining".)