Price of flat, one brother’s acquired (4,5)
I believe the answer is:
even money
'price' is the definition.
(type of bet)
'flat one brother's acquired' is the wordplay.
'flat' becomes 'even' (similar in meaning).
'brother' becomes 'my' (both are expressions of shock).
'acquired' means one lot of letters goes inside another.
'one' put within 'my' is 'money'.
'even'+'money'='EVEN MONEY'
'of' is the link.
(Other definitions for even money that I've seen before include "Odds offering an equal chance of winning or losing" , "Betting odds offering chance to win the amount wagered" , "Kind of bet where you stand to win or lose the sum wagered" , "likely to happen!" , "A good chance" .)