Sacked if upsetting socialist (5)
I believe the answer is:
fired
'sacked' is the definition.
(both can mean dismissed from a job)
'if upsetting socialist' is the wordplay.
'upsetting' says the letters should be written in reverse.
'socialist' becomes 'red' (socialism is often associated with the colour red).
'if' written backwards gives 'fi'.
'fi'+'red'='FIRED'
(Other definitions for fired that I've seen before include "with P45?" , "got rid of" , "Discharged" , "Aroused" , "(Of pottery) baked" .)