Just — not bad (4)
I believe the answer is:
fair
This clue is a double definition.
'just' is the first definition.
(just can mean fair in the sense of showing justice)
'not bad' is the second definition.
(fair can mean of decent quality)
(Other definitions for fair that I've seen before include "square" , "Reasonable; pretty" , "Show - just - lovely" , "Pretty; impartial" , "... enough! - that's reasonable" .)