Most excellent journey on horseback, with one leg on either side (8)
I believe the answer is:
bestride
'with one leg on either side' is the definition.
(as in being bestride a horse)
'most excellent journey on horseback' is the wordplay.
'most excellent' becomes 'best' (the very best).
'journey on horseback' becomes 'ride' (ride can mean to journey on horseback).
'best'+'ride'='BESTRIDE'
(Other definitions for bestride that I've seen before include "get a legover?" , "Mounted" , "Do this to horse" , "Stand over protectively" , "Dominate" .)