A nasty one could upset some countrymen (4)
I believe the answer is:
turn
'upset some countrymen' is the definition.
'turn' can be an answer for 'upset' (thesaurus). I am not sure about the 'some countrymen' bit.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for turn that I've seen before include "Go off - performance" , "Revolution by runt" , "Start" , "Go off - act" , "Revolve - go off" .)