Bonus feature (5)
'bonus feature' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for extra that I've seen before include "Ancillary" , "Especially" , "Something additional or bit player in movie" , "Additional - run - actor - newspaper edition" , "Way of scoring at cricket" .)
(Other definitions for add-on that I've seen before include "Accessory device" , "Feature bolted on to a standard model or package" , "Extra bit" , "Computer peripheral" , "Computer optional extra" .)