Horse-racing windfall leaving one taken aback (4)
I believe the answer is:
turf
'horse-racing' is the definition.
('the turf' can refer to horseracing as a sport)
'windfall leaving one taken aback' is the wordplay.
'windfall' becomes 'fruit' (windfall can refer to a fruit blown down by wind).
'leaving' is a deletion indicator.
'one' becomes 'i' (Roman numeral).
'taken aback' is a reversal indicator.
'fruit' with 'i' removed is 'frut'.
'frut' backwards is 'TURF'.
(Other definitions for turf that I've seen before include "Grass; slab of peat" , "Patch" , "Gang's territory" , "Surface of matted grass - horse racing" , "Gang territory" .)