It's goodbye to Dai, maybe, who's so backward (5)
I believe the answer is:
adios
'it's goodbye' is the definition.
(synonyms)
'who's so backward' is the wordplay.
'who' becomes 'adi' (I can't justify this - if you can you should give a lot more credence to this answer).
'backward' says the letters should be written backwards.
'so' backwards is 'os'.
'adi'+'os'='ADIOS'
'to dai maybe' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for adios that I've seen before include "Au revoir, amigo!" , "so long abroad" , "Goodbye to Spain" , "See you in Ibiza" , "Good-bye (Sp.)" .)