So long, and thanks again (2-2)
I believe the answer is:
ta-ta
'so long' is the definition.
(both are ways of saying goodbye)
'thanks again' is the wordplay.
I cannot quite see how this works, but
'thanks' could be 'ta' (UK informal version of 'thank you') and 'ta' is present in the answer.
The remaining letters 'ta' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'and' is the link.
Can you help me to learn more?
(Other definitions for ta-ta that I've seen before include "A Farewell" , "Bye!" , "Adieu" , "Bye-bye" , "Cheerio" .)