PM a Conservative joining fist-fight among staff (9)
I believe the answer is:
macmillan
'pm' is the definition.
(I have seen 'former PM' mean 'macmillan' so perhaps 'pm' could also mean 'macmillan')
'a conservative joining fist-fight among staff' is the wordplay.
'a conservative joining' becomes 'ac' (I can't explain this - if you can you should give a lot more credence to this answer).
'fist fight' becomes 'mill' (I've seen this before).
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'staff' becomes 'man' (as in staffing or manning a particular place).
'ac'+'mill'='acmill'
'acmill' going within 'man' is 'MACMILLAN'.
Can you help me to learn more?
(Other definitions for macmillan that I've seen before include "Harold -, PM 1957 to 63" , "PM after Eden" , "Prime Minister" , "Conservative leader" , "Harold -, former PM" .)