They may well be only human (6)
I believe the answer is:
bipeds
'they' is the definition.
Both the answer and definition are plural nouns.
Perhaps you can see a link between them that I can't see?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for bipeds that I've seen before include "creatures like us" , "Upright creatures" , "Animals that walk on two feet" , "Two-footed animals" , "Two-legged animals" .)