Word of gratitude repeated for so long (2-2)
I believe the answer is:
ta-ta
'so long' is the definition.
(both are ways of saying goodbye)
'word of gratitude repeated' is the wordplay.
'word of gratitude' becomes 'ta' (informal way of saying thank you).
'repeated' indicates the doubling of some letters.
'ta' doubled is 'TA-TA'.
'for' acts as a link.
(Other definitions for ta-ta that I've seen before include "Bye-bye" , "Adieu" , "Bye!" , "A Farewell" , "Cheerio" .)