Nathaniel's buttocks (5)
I believe the answer is:
nates
'buttocks' is the definition.
(I've seen this before)
'nathaniel's' is the wordplay.
I cannot really understand how this works, but
'nathaniel' could be 'nat' (I've seen this in other clues) and 'nat' is found within the answer.
The remaining letters 'es' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nates that I've seen before include "bottom" , "Buttocks" , "behind" .)