Mounted on (7)
I believe the answer is:
astride
'mounted on' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for astride that I've seen before include "Standing over amid tirades" , "Not side saddle" , "With feet apart" , "With legs apart, emitting tirades" , "With legs stretched apart - tirades (anag)" .)