Supplementary component (9)
(Other definitions for accessory that I've seen before include "additional" , "Supplementary component of clothing" , "Someone who helps another commit a crime" , "Item such as a handbag that goes with a dress" , "Confederate" .)
(Other definitions for add-on that I've seen before include "Computer optional extra" , "Optional item as supplement or increment" , "Feature bolted on to a standard model or package" , "Feature supplementing standard model" , "Extra peripheral item" .)
(Other definitions for addon that I've seen before include "something extra" , "Extra charge" .)