Financial supporter (6)
'financial supporter' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for backer that I've seen before include "Angel" , "A supporter or a betting man" , "Financial supporter" , "Person supporting another financially" , "Sponsor" .)
(Other definitions for benefactor that I've seen before include "Person giving to a good cause" , "charity supporter?" , "One provides bread" , "Patronising sort" , "Patron, sponsor" .)
(Other definitions for sponsor that I've seen before include "One offering support" , "Patron; promote" , "Supporter or champion" , "One who pays costs for event" , "Promoter; funder" .)