Accounted for former priest backing rejection (9)
I believe the answer is:
explained
'accounted for' is the definition.
(to account for something is to explain it)
'former priest backing rejection' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'priest' becomes 'p' (ecclesiastical abbreviation).
'backing' is a reversal indicator.
'rejection' becomes 'denial' (rejection of a claim).
'denial' backwards is 'lained'.
'ex'+'p'+'lained'='EXPLAINED'
(Other definitions for explained that I've seen before include "Made clear, gave reasons for" , "opened up" , "Made comprehensible" , "Elucidated, clarified" .)