Most of the sheet written by an unbeliever (5)
I believe the answer is:
pagan
'unbeliever' is the definition.
(I've seen this before)
'most of the sheet written by an' is the wordplay.
'most of' means to remove the last letter.
'the sheet' becomes 'page' (page is a kind of sheet).
'written by' says to put letters next to each other.
'page' with its final letter removed is 'pag'.
'pag'+'an'='PAGAN'
(Other definitions for pagan that I've seen before include "Heathen; non-believer" , "Person without any conventional religion" , "Non-mainstream religious believer" , "Unchristian" , "Infidel" .)