Just terrible after fifty (6)
I believe the answer is:
lawful
'just' is the definition.
(showing justice)
'terrible after fifty' is the wordplay.
'terrible' becomes 'awful' (synonyms).
'after' says to put letters next to each other.
'fifty' becomes 'l' (Roman numeral).
'awful' after 'l' is 'LAWFUL'.
(Other definitions for lawful that I've seen before include "Legally permitted" , "Legally permissible" , "Allowed" , "Not illegal" , "Licit" .)