A northern English vale runs up biblical mountain (6)
I believe the answer is:
ararat
'biblical mountain' is the definition.
(I've seen this before)
'a northern english vale runs up' is the wordplay.
'northern english vale' becomes 'ta-ra' (goodbye in the north of England).
'runs' becomes 'r' (cricket abbreviation).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'tara'+'r'='tarar'
'tarar' in reverse letter order is 'rarat'.
'a'+'rarat'='ARARAT'
(Other definitions for ararat that I've seen before include "Mount on which Noah's ark grounded" , "Mount where Noah's Ark came to rest" , "original dry dock" , "Highest mountain in Turkey" , "Biblical mountains" .)