Up in London a gaping heathen (5)
I believe the answer is:
pagan
'heathen' is the definition.
(both can mean an irreligious person)
'up in london a gaping' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in' indicates the answer is hidden within the clue.
'nagap' is hidden in the letters of 'london a gaping'.
'nagap' reversed gives 'PAGAN'.
(Other definitions for pagan that I've seen before include "One not accepting your god" , "One who does not acknowledge a god" , "Polytheist" , "One who does not acknowledge your god(s)" , "A heathen, not a Christian" .)