They have their planet, rather like us (4)
I believe the answer is:
'us' is the definition.
Although both the answer and definition are plural nouns, I don't see how they can define each other.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for apes that I've seen before include "Copies the primates?" , "Peas (anag)" , "Imitates in every aspect" , "Species of primate" , "Imitates like primates" .)