Slope that's good for breaking in horse (5)
I believe the answer is:
ridge
'slope' is the definition.
'good for breaking in horse' is the wordplay.
'good for' becomes 'g' (I've seen this in other clues).
'breaking in' is an insertion indicator.
'horse' becomes 'ride' (**).
'g' inserted into 'ride' is 'RIDGE'.
'that's' acts as a link.
(Other definitions for ridge that I've seen before include "Long narrow natural elevation" , "Long, narrow range of hills" , "Long, narrow hilltop" , "there's a lot of pressure in it" , "Rooftop" .)