A way to go on horseback - with legs apart (7)
I believe the answer is:
astride
'with legs apart' is the definition.
(as in sitting astride a horse)
'a way to go on horseback' is the wordplay.
'way' becomes 'st' (abbreviation for street).
'to go on horseback' becomes 'ride' (I've seen this before).
'a'+'st'+'ride'='ASTRIDE'
(Other definitions for astride that I've seen before include "With legs on each side" , "Not side saddle" , "Passing across" , "Extending across" , "on horseback" .)