He doesn't want people to keep their hair on! (6)
I believe the answer is:
barber
'he doesn't want people to keep their hair on' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he doesn't want' is the wordplay.
'he' becomes 'Bar'.
'doesn't want' becomes 'ber' (I can't justify this - if you can you should believe this answer much more).
'bar'+'ber'='BARBER'
Can you help me to learn more?
(Other definitions for barber that I've seen before include "One who cuts hair" , "American composer; hairdresser" , "Hairdresser who cuts and shaves" , "one to profit from the cuts?" , "Hairdresser for men - musically from Seville" .)