Beginnings of avalanche — rush back in time to shelter (7)
I believe the answer is:
harbour
'shelter' is the definition.
(harbouring is a kind of sheltering)
'beginnings of avalanche rush back in time' is the wordplay.
'beginnings of' indicates taking the first letters.
'in' is an insertion indicator.
'time' becomes 'hour' (hour is a kind of time**).
The first letters of 'avalanche rush back' is 'arb'.
'arb' going into 'hour' is 'HARBOUR'.
'to' acts as a link.
(Other definitions for harbour that I've seen before include "Conceal" , "Shelter for shipping" , "Dock" , "Hide - entertain - port" , "Sheltered anchorage" .)