Additional, ancillary (13)
I believe the answer is:
supplementary
I believe this clue is a double definition.
'additional' is the first definition.
(I've seen this before)
'ancillary' is the second definition.
(I've seen this before)
(Other definitions for supplementary that I've seen before include "Extra" , "Added to complete or make up a deficiency" , "Ancillary" , "Additional" .)