Add sign: top house is luxurious (5)
I believe the answer is:
plush
'luxurious' is the definition.
(I've seen this before)
'add sign top house' is the wordplay.
I cannot really see how this works, but
'add' could be 'plus' (I've seen this before) and 'plus' is found within the answer.
'house' could be 'h' and 'h' is found in the answer.
No letters remain.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for plush that I've seen before include "Fabric with a long velvety pile" , "Fabric; richly luxurious" , "Cut pile, softer than velvet" , "Rich fabric" .)