Discontent in leaving EU - insane rubbish! (6)
I believe the answer is:
unease
'discontent' is the definition.
(thesaurus)
'in leaving eu insane rubbish' is the wordplay.
'leaving' is a deletion indicator.
'rubbish' indicates an anagram.
'eu'+'insane'='euinsane'
'euinsane' with 'in' taken away is 'eusane'.
'eusane' anagrammed gives 'UNEASE'.
(Other definitions for unease that I've seen before include "State of anxiety" , "Anxiety, discontent" , "Discomfort or apprehension" , "Disquiet" , "State of worry and lack of confidence" .)