Snowslip burying a northern church in a dale (9)
I believe the answer is:
avalanche
'snowslip' is the definition.
(type of avalanche)
'burying a northern church in a dale' is the wordplay.
'burying' indicates putting letters inside.
'a' becomes 'an'.
'northern church' becomes 'ch' (common abbreviation. I am not sure about the 'northern' bit.).
'in' is an insertion indicator.
'dale' becomes 'vale' (both can mean a valley).
'a'+'vale'='avale'
'ch' placed into 'avale' is 'avalche'.
'an' put within 'avalche' is 'AVALANCHE'.
(Other definitions for avalanche that I've seen before include "Sudden descent of massed snow etc" , "Slip that could be very cold" , "Large snow or rock fall" , "Snowslip" , "This dangerous flow from some peaks" .)