Fill puzzle (6)

I believe the answer is:

riddle

I believe this is a double definition.

'fill' is the first definition.
(thesaurus)

'puzzle' is the second definition.
(a riddle is a puzzle or problem)

(Other definitions for riddle that I've seen before include "Brain-teaser and coarse sieve" , "Ingenious puzzle - sieve" , "Perforate with gunshot - poser" , "it's full of holes" , "Strain" .)

I've seen this clue in The Telegraph.

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