It stumps our cousin: he did you? (9)
I believe the answer is:
araucaria
'it stumps our cousin' is the definition.
The definition suggests a singular noun which matches the answer.
'he did you?' is the wordplay.
I cannot really see how this works, but
'he' could be 'car' and 'car' is present in the answer.
'you?' could be 'u' (homophones) and 'u' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for araucaria that I've seen before include "Evergreen" , "puzzle for primates?" , "Monkey puzzle tree (or Chile pine)" , "Coniferous tree" , "Monkey puzzle tree - Guardian cryptic crossword setter" .)