Major sale is quite a thing (3,4)
I believe the answer is:
big deal
'quite a thing' is the definition.
I can't tell whether this definition defines the answer.
'major sale' is the wordplay.
'major' becomes 'big' (I've seen this in other clues).
'sale' becomes 'deal' (I can't justify this - if you can you should believe this answer much more).
'big'+'deal'='BIG DEAL'
'is' is the link.
Can you help me to learn more?
(Other definitions for big deal that I've seen before include "Handing out lots of cards" , "doesn't impress me at all" , "Scornful response to boast (colloq.)" , "In slang, I'm not impressed - by large wood?" , "sarcastic response" .)