Indicating an addition that's almost luxurious (4)
I believe the answer is:
plus
'indicating an addition that's' is the definition.
I can't tell whether this definition defines the answer.
'almost luxurious' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'luxurious' becomes 'plush' (I've seen this in another clue).
'plush' with its final letter taken off is 'PLUS'.
Can you help me to learn more?
(Other definitions for plus that I've seen before include "Advantage; also" , "Add" , "asset" , "Positive sign" , "being a little cross" .)