An advantage, being endlessly luxurious (4)
I believe the answer is:
plus
'an advantage' is the definition.
(a plus is an advantage or benefit)
'endlessly luxurious' is the wordplay.
'endlessly' means to remove the last letter.
'luxurious' becomes 'plush' (I've seen this before).
'plush' with its final letter taken away is 'PLUS'.
'being' acts as a link.
(Other definitions for plus that I've seen before include "Add" , "Furthermore" , "Additional" , "Positive - addition" , "Advantage; also" .)