Money young lady invested in horse’s strap (10)
I believe the answer is:
martingale
'strap' is the definition.
(I've seen this before)
'money young lady invested in horse's' is the wordplay.
'money' becomes 'tin' (I've seen this before).
'young lady' becomes 'gal' ('gal' can be a synonym of 'young lady'**).
'invested in' is an insertion indicator.
'horse' becomes 'mare' (mare is a kind of horse).
'tin'+'gal'='tingal'
'tingal' put inside 'mare' is 'MARTINGALE'.
(Other definitions for martingale that I've seen before include "strap" , "Part of a horse's harness" , "Going for broke at the casino" , "wooden support" , "bit of tack" .)