Backer or client (6)
I believe the answer is:
patron
I believe this clue is a double definition.
'backer' is the first definition.
(I've seen this in another clue)
'client' is the second definition.
(I know that patron is a type of client)
(Other definitions for patron that I've seen before include "Patrick for Ireland, for example" , "Restaurant customer" , "Regular customer or sponsor" , "Regular customer; backer" , "One gives financial or other support, to an artist say" .)