Style of decoration reflected hollow decadence (4)
I believe the answer is:
mode
'style' is the definition.
(I know that style can be written as mode)
'decoration reflected hollow decadence' is the wordplay.
'decoration' becomes 'OM' (Order of Merit).
'reflected' says the letters should be written in reverse (letters reflected back).
'hollow' says to hollow out the word (remove centre letters) (the word is hollowed out).
'decadence' with its middle removed is 'de'.
'om' reversed gives 'mo'.
'mo'+'de'='MODE'
'of' is the link.
(Other definitions for mode that I've seen before include "Style of dome" , "Way of operating" , "Way, method" , "Mistake" , "Way of doing something" .)