Luxury’s endless advantage (4)
I believe the answer is:
plus
'advantage' is the definition.
(a plus is an advantage or benefit)
'luxury's endless' is the wordplay.
'luxury' becomes 'plush' (plush can mean luxury or deluxe).
'endless' means to remove the last letter.
'plush' with its final letter taken off is 'PLUS'.
(Other definitions for plus that I've seen before include "being a little cross" , "And also; a bonus" , "Furthermore" , "Positive quality" , "The opposite of minus" .)