A court visit upset campaigner (8)
I believe the answer is:
activist
'campaigner' is the definition.
(I've seen this before)
'a court visit upset' is the wordplay.
'court' becomes 'ct'.
'upset' indicates an anagram.
'visit' anagrammed gives 'ivist'.
'a'+'ct'+'ivist'='ACTIVIST'
(Other definitions for activist that I've seen before include "Militant" , "Vigorous political campaigner" , "One who plays a vigorous role, especially in politics" , "Human dynamo" , "Cause promoter" .)