Article by supporter backing backer (5)
I believe the answer is:
angel
'backer' is the definition.
('angel' can be a synonym of 'backer')
'article by supporter backing' is the wordplay.
'article' becomes 'an' (indefinite article).
'by' means one lot of letters go next to another.
'supporter' becomes 'leg'.
'backing' says the letters should be written in reverse.
'leg' backwards is 'gel'.
'an'+'gel'='ANGEL'
(Other definitions for angel that I've seen before include "One usually obliging" , "Very helpful person" , "Divine messenger" , "Seraph or cherub?" , "Guardian spirit; theatre investor" .)