Go on horseback astride good saddle (5)
I believe the answer is:
ridge
'saddle' is the definition.
(I've seen this before)
'go on horseback astride good' is the wordplay.
'go on horseback' becomes 'ride' (I've seen this before).
'astride' indicates putting letters inside.
'good' becomes 'g' (abbreviation).
'ride' going around 'g' is 'RIDGE'.
(Other definitions for ridge that I've seen before include "Wrinkle" , "Raised edge" , "Line of raised ground" , "Narrow hill-range" , "Escarpment" .)