Article upset stage show's backer (5)
I believe the answer is:
angel
'show's backer' is the definition.
(an angel is a financial backer)
'article upset stage' is the wordplay.
'article' becomes 'an' (indefinite article).
'upset' is a reversal indicator.
'stage' becomes 'leg' (a leg is a stage of a competition).
'leg' back-to-front is 'gel'.
'an'+'gel'='ANGEL'
(Other definitions for angel that I've seen before include "Ministering spirit" , "lady (very kind)" , "Heavenly body" , "one might be recording" , "somewhere on the Northern Line" .)