No opening for group court case (6)
I believe the answer is:
action
'court case' is the definition.
(a legal action)
'no opening for group' is the wordplay.
'no opening for' means to remove the first letter.
'group' becomes 'faction'.
'faction' with its initial letter removed is 'ACTION'.
(Other definitions for action that I've seen before include "exploit" , "instruction from director" , "Legal proceeding brought by one party against another" , "Deed, maybe battle" , "Accomplishment" .)