An unexpected trifle -- a riddle, perhaps (6)
I believe the answer is:
filter
'a riddle perhaps' is the definition.
(riddle is a kind of filter)
'an unexpected trifle' is the wordplay.
'an unexpected' indicates anagramming the letters.
'trifle' is an anagram of 'FILTER'.
(Other definitions for filter that I've seen before include "Strainer; a traffic light" , "Pass through screen to remove impurities" , "Screen out unwanted material" , "Remover of impurities" , "Device which allows fluids but not solids to pass through" .)