His friends may deprive him of his ultimate right (5)
I believe the answer is:
peter
'his friends may deprive him of his' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'ultimate right' is the wordplay.
'ultimate' becomes 'pete' (I can't justify this - if you can you should give a lot more credence to this answer).
'right' becomes 'r' (common abbreviation).
'pete'+'r'='PETER'
Can you help me to learn more?
(Other definitions for peter that I've seen before include "emperor" , "- Sellers, twentieth century English comedian" , "' - - Pan' by Barrie" , "and 14 dn Latest actor to play Doctor Who" , "- Pan, eternal youth" .)