The last thing said is “We’re tumbling in an avalanche” (8)
I believe the answer is:
farewell
'the last' is the definition.
(thesaurus)
'we're tumbling in an avalanche' is the wordplay.
'tumbling' indicates anagramming the letters.
'in' is an insertion indicator.
'an avalanche' becomes 'fall' (I can't explain this - if you can you should believe this answer much more).
'were' anagrammed gives 'rewe'.
'rewe' placed within 'fall' is 'FAREWELL'.
'thing said is' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for farewell that I've seen before include "Goodbye, hope you get on" , "Archaic goodbye - good luck" , "It may be said when 15" , "Goodbye, cheerio" , "Archaic goodbye or leave-taking" .)