A new church established in a valley in the fall (9)
I believe the answer is:
avalanche
'the fall' is the definition.
(I've seen this before)
'a new church established in a valley' is the wordplay.
'a new' becomes 'an' (I've seen this in other clues).
'church' becomes 'ch' (common abbreviation).
'established in' indicates putting letters inside.
'valley' becomes 'vale' ('vale' can be a synonym of 'valley'**).
'an'+'ch'='anch'
'a'+'vale'='avale'
'anch' placed within 'avale' is 'AVALANCHE'.
'in' is the link.
(Other definitions for avalanche that I've seen before include "Sudden descent of massed snow etc" , "Landslide of snow" , "This dangerous flow from some peaks" , "Deluge" , "sudden rush" .)