Just pants put on line (6)
I believe the answer is:
lawful
'just' is the definition.
(showing justice)
'pants put on line' is the wordplay.
'pants' becomes 'awful' (I've seen this before).
'put on' means one lot of letters go next to another.
'line' becomes 'l' (used when specifying particular lines from a poem).
'awful' after 'l' is 'LAWFUL'.
(Other definitions for lawful that I've seen before include "Allowed" , "Legitimate" , "Right!" , "Legally permissible" , "Legal" .)