Windfall maybe, one I’m surprised about (5)
I believe the answer is:
money
'windfall maybe' is the definition.
(windfall can mean money received unexpectedly)
'one i'm surprised about' is the wordplay.
'i'm surprised' becomes 'my' ('my' is an expression of surprise).
'about' means one lot of letters goes inside another.
'one' going within 'my' is 'MONEY'.
(Other definitions for money that I've seen before include "It's the root of all evil" , "Legal tender" , "See 7" , "Cash, notes" , "Mazuma, ackers or dosh" .)