He repeats himself, yet probably keeps mum (4)
I believe the answer is:
papa
'he repeats himself yet probably keeps mum' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he repeats' is the wordplay.
'he' becomes 'pa' (I can't explain this - if you can you should believe this answer much more).
'repeats' shows that letters should be duplicated.
'pa' duplicated is 'PAPA'.
Can you help me to learn more?
(Other definitions for papa that I've seen before include "The old man" , "Comms code for 'P'" , "Father; letter P" , "Daddy]" , "one after Oscar" .)