Horse for riding (5)
(Other definitions for steed that I've seen before include "ride" , "Powerful horse" , "A mount (poet.)" , "Swift horse" , "(Poetic) horse" .)
'horse for riding' is the definition.
(horse that can be mounted)
This is the entire clue.
(Other definitions for mount that I've seen before include "Take < a back" , "Horse; climb" , "Get on - horse" , "Climb, stairs say" , "put on launch" .)