I invested in horse over win (4)
I believe the answer is:
gain
'win' is the definition.
(I know that win can be written as gain)
'i invested in horse over' is the wordplay.
'invested in' means one lot of letters goes inside another.
'horse' becomes 'nag' (term for an old horse).
'over' is a reversal indicator.
'i' put inside 'nag' is 'niag'.
'niag' written backwards gives 'GAIN'.
(Other definitions for gain that I've seen before include "reward" , "Obtain; benefit" , "Pick up" , "Benefit; put on (weight)" , "Benefit; increase in value" .)