Guardian article by setter (5)
I believe the answer is:
angel
'guardian' is the definition.
(guardian angel)
'article by setter' is the wordplay.
'article' becomes 'an' (indefinite article).
'by' is a charade indicator (letters next to each other).
'setter' becomes 'gel' (something which sets).
'an'+'gel'='ANGEL'
(Other definitions for angel that I've seen before include "Theatre financier" , "Holy being" , "somewhere in Islington" , "Old coin; theatre investor" , "Glean (anag.)" .)