Mount carrying good saddle (5)
I believe the answer is:
ridge
'saddle' is the definition.
(I've seen this before)
'mount carrying good' is the wordplay.
'mount' becomes 'ride' (riding is a kind of mounting**).
'carrying' means one lot of letters goes inside another.
'good' becomes 'g' (abbreviation).
'ride' enclosing 'g' is 'RIDGE'.
(Other definitions for ridge that I've seen before include "Long, narrow range of hills" , "Long narrow crest" , "Long raised band" , "Feature of mountain" , "Line of raised ground" .)