Appends (4)
'appends' is the definition.
('add' can be a synonym of 'append')
This is all the clue.
(Other definitions for adds that I've seen before include "Appends; Totals" , "Sums, totals" , "Contributes further" , "Tots up" , "Finds the sum (4)," .)
'appends' is the definition.
('tack on' can be a synonym of 'append')
This is the entire clue.
(Another definition for tacks on that I've seen is " Fixes upon".)