Add as a supplement (6)
I believe the answer is:
append
'add as a supplement' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for append that I've seen before include "Add, annex" , "Affix to the end" , "Add (to end of text)" , "Napped (anag)" , "Add (at the back of a document)" .)