Cheerio to you I say, after this time that is (5)
I believe the answer is:
adieu
'cheerio to you i say after this time that is' is the definition.
'adieu' can be an answer for 'cheerio' (synonyms). I'm unsure of the rest of the definition.
'say after this time that is' is the wordplay.
'say' becomes 'u' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'after' says to put letters next to each other.
'this time' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'ad'+'ie'='adie'
'u' after 'adie' is 'ADIEU'.
Can you help me to learn more?
(Other definitions for adieu that I've seen before include "cheers" , "Parting greeting" , "Goodbye, mademoiselle" , "Goodbye to the French" , "Auf wiedersehen" .)