Riddle good seen in fantastic anime (6)
I believe the answer is:
enigma
'riddle' is the definition.
(I know that riddle can be written as enigma)
'good seen in fantastic anime' is the wordplay.
'good' becomes 'g' (abbreviation).
'seen in' indicates putting letters inside.
'fantastic' indicates anagramming the letters.
'anime' with letters rearranged gives 'enima'.
'g' going into 'enima' is 'ENIGMA'.
(Other definitions for enigma that I've seen before include "Mysteriously puzzling person or thing" , "closed book" , "Puzzle found in gem, Ian" , "Coding device" , "Mysterious person/thing" .)