Doubting it with number reversed in account (8)
I believe the answer is:
agnostic
'doubting' is the definition.
(someone unsure on the topic of religion)
'it with number reversed in account' is the wordplay.
'with' says to put letters next to each other.
'number' becomes 'song' (musical number).
'reversed' says the letters should be written backwards.
'in' is an insertion indicator.
'account' becomes 'ac' (abbreviation).
'it'+'song'='itsong'
'itsong' in reverse letter order is 'gnosti'.
'gnosti' placed inside 'ac' is 'AGNOSTIC'.
(Other definitions for agnostic that I've seen before include "Person who thinks nothing can be known about God" , "Sceptic - unbelieving" , "Believing that nothing can be known about the existence of God" , "Coasting with unbeliever" , "Cost gain for one who holds no spiritual belief" .)