Great trade? Who cares (3,4)
I believe the answer is:
big deal
'cares' is the definition.
The definition and answer are not the same part of speech.
'great trade?' is the wordplay.
'great' becomes 'big' (synonyms).
'trade?' becomes 'deal' ('deal' can be a synonym of 'trade').
'big'+'deal'='BIG DEAL'
'who' is the link.
Can you help me to learn more?
(Other definitions for big deal that I've seen before include "Exclamation of scorn, belittling a claim" , "Scornful response to boast (colloq.)" , "doesn't impress me at all" , "Significant agreement - so what!" , "Potential slam hand at bridge" .)