Twenty per cent off expensive asset (4)
I believe the answer is:
plus
'asset' is the definition.
('plus' can be a synonym of 'asset')
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for plus that I've seen before include "mark for arithmetic" , "And, arithmetically" , "Positive - addition" , "Positive and not a negative" , "asset" .)