Sacked if up to Communist (5)
I believe the answer is:
fired
'sacked' is the definition.
(both can mean dismissed from a job)
'if up to communist' is the wordplay.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'to communist' becomes 'red' (red can informally mean communist).
'if' reversed gives 'fi'.
'fi'+'red'='FIRED'
(Other definitions for fired that I've seen before include "(Of pottery) baked" , "got rid of" , "Set ablaze" , "Sacked or ignited" , "Stimulated; dismissed" .)