Within reason, lying can be just (4)
I believe the answer is:
only
'just' is the definition.
('only' can be a synonym of 'just')
'within reason lying can' is the wordplay.
'within' indicates a hidden word.
'ONLY' can be found hidden inside 'reason lying can'.
'be' is the link.
(Other definitions for only that I've seen before include "Merely; single" , "Solely; except that" , "Solely, lone" , "Except that; single" , "Simply" .)