Just terrible alongside Labour leader (6)
I believe the answer is:
lawful
'just' is the definition.
(thesaurus)
'terrible alongside labour leader' is the wordplay.
'terrible' becomes 'awful' ('awful' can be a synonym of 'terrible'**).
'alongside' means one lot of letters go next to another.
'leader' suggests taking the first letters.
The first letter of 'labour' is 'l'.
'awful' after 'l' is 'LAWFUL'.
(Other definitions for lawful that I've seen before include "Licit" , "Legally permissible" , "Not illegal" , "Legally permitted" , "Allowed" .)