Just article in tree (4)
I believe the answer is:
fair
'just' is the definition.
(just can mean fair in the sense of showing justice)
'article in tree' is the wordplay.
'article' becomes 'a' (English indefinite article).
'in' means one lot of letters goes inside another.
'tree' becomes 'fir' (fir tree is a kind of tree).
'a' inserted inside 'fir' is 'FAIR'.
(Other definitions for fair that I've seen before include "likely" , "Just; fete" , "Beautiful like maiden of old" , "Even-handed travelling show?" , "Light; entertainment" .)