Riddling monster of Greek mythology (6)
I believe the answer is:
sphinx
'riddling monster of greek mythology' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for sphinx that I've seen before include "puzzler" , "Riddling monster, top half a woman, bottom half a lion" , "Inscrutable character" , "Egyptian 'lion-man'" , "mythical monster" .)