PC entered, upset with previous parallel case (9)
I believe the answer is:
precedent
'previous parallel case' is the definition.
'precedent' can be an answer for 'case' (I have seen 'previous case' mean 'precedent' so perhaps 'case' could also mean 'precedent'). I'm not sure about the 'previous parallel' bit.
'pc entered upset' is the wordplay.
'upset' indicates an anagram.
'pc'+'entered'='pcentered'
'pcentered' anagrammed gives 'PRECEDENT'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for precedent that I've seen before include "Previous instance (as justification)" , "Previous example" , "exemplary court case" , "Previous case cited in court" , "An earlier example serving as a guide" .)