Just one has gone in a long way (4)
I believe the answer is:
fair
'just' is the definition.
(just can mean fair in the sense of showing justice)
'one has gone in a long way' is the wordplay.
'one has gone' becomes 'i' (I can't justify this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'a long way' becomes 'far' (I've seen this in another clue).
'i' placed into 'far' is 'FAIR'.
Can you help me to learn more?
(Other definitions for fair that I've seen before include "Sign following Pisces" , "the place for a roundabout" , "Average" , "Traders' gathering" , "Exhibition of commercial or industrial goods" .)