Ancient Greeks were riddled by it (6)
I believe the answer is:
sphinx
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for sphinx that I've seen before include "ancient monument" , "Riddle-asking monster of Greek mythology" , "Inscrutable character" , "'Riddling monster, half-woman, half-lion (6)'" , "Monster of Egyptian statuary" .)