Cheerio: earl flew out (8)
I believe the answer is:
farewell
'cheerio' is the definition.
(I know that cheerio can be written as farewell)
'earl flew out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'earl'+'flew'='earlflew'
'earlflew' is an anagram of 'FAREWELL'.
(Other definitions for farewell that I've seen before include "Archaic goodbye or leave-taking" , "vale" , "Say goodbye" , "Departure" , "Valediction" .)