Margin authentic and celestial (8)
I believe the answer is:
sidereal
'celestial' is the definition.
(thesaurus)
'margin authentic' is the wordplay.
'margin' becomes 'side' (I've seen this in another clue).
'authentic' becomes 'real' (I've seen this before).
'side'+'real'='SIDEREAL'
'and' acts as a link.
(Other definitions for sidereal that I've seen before include "Of the stars or constellations" , "Realised (anag) -- relating to the stars" , "Of or measured by the stars" , "involving stars" , "Full of stars" .)