Unbeliever opening up about new beginnings (5)
I believe the answer is:
pagan
'unbeliever' is the definition.
(I've seen this before)
'opening up about new beginnings' is the wordplay.
'opening' becomes 'gap' (gap is a kind of opening).
'up' is a reversal indicator (in down clue: letters go upwards).
'beginnings' says to take the initial letters.
The initial letters of 'about new' is 'an'.
'gap' in reverse letter order is 'pag'.
'pag'+'an'='PAGAN'
(Other definitions for pagan that I've seen before include "One not accepting your god" , "Polytheist" , "One who does not acknowledge your god(s)" , "Pantheistic" , "Not conventionally religious" .)