Business is booming, so get something more expensive (5,2)
I believe the answer is:
trade up
'get something more expensive' is the definition.
'business is booming' is the wordplay.
'business' becomes 'trade' (trade is a kind of business**).
'is booming' becomes 'up'.
'trade'+'up'='TRADE UP'
'so' is the link.
(Other definitions for trade up that I've seen before include "Move to a larger property" , "Buy a bigger property" , "Buy larger house" , "Buy a better model" .)