Expression of surprise about one's wealth (5)
I believe the answer is:
money
'wealth' is the definition.
(I know that wealth can be written as money)
'expression of surprise about one's' is the wordplay.
'expression of surprise' becomes 'my' ('my!' is an expression of surprise).
'about' indicates putting letters inside.
'my' placed around 'one' is 'MONEY'.
(Other definitions for money that I've seen before include "tin" , "Currency; wealth" , "'A fool and his.....are soon parted'" , "Lucre" , "It's the root of all evil" .)