Reportedly saw a creature with eight legs (6)
I believe the answer is:
spider
'with eight legs' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for spider that I've seen before include "Snooker equipment" , "Rest for a cue (with eight legs?)" , "Arthropod; snooker rest" , "Snooker-cue rest" , "Harvester" .)