Break down wall — free! Goodbye! (8)
I believe the answer is:
farewell
'goodbye' is the definition.
(I've seen this before)
'break down wall free' is the wordplay.
'break down' indicates an anagram.
'wall'+'free'='wallfree'
'wallfree' anagrammed gives 'FAREWELL'.
(Other definitions for farewell that I've seen before include "Valediction" , "Goodbye, cheerio" , "Wish on leave-taking" , "Goodbye, hope you get on" , "Goodbye, adieu, so long" .)