Happiness initially is only for so long (7)
I believe the answer is:
cheerio
'for so long' is the definition.
(both can mean goodbye)
'happiness initially is only' is the wordplay.
'happiness' becomes 'cheer'.
'initially' suggests taking the first letters.
The first letters of 'is only' is 'io'.
'cheer'+'io'='CHEERIO'
(Other definitions for cheerio that I've seen before include "Final message" , "for so long!" , "See you" , "good health" , "A final word" .)