Major sale is quite a thing (3,4)
I believe the answer is:
'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).
'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" .)