Just a tree outside (4)
I believe the answer is:
fair
'just' is the definition.
(just can mean fair in the sense of showing justice)
'a tree outside' is the wordplay.
'tree' becomes 'fir' (fir is a kind of tree).
'outside' is an insertion indicator.
'a' placed inside 'fir' is 'FAIR'.
(Other definitions for fair that I've seen before include "honest" , "the place for a roundabout" , "Treating people equally" , "All clues should be" , "Impartial, so-so" .)