With one leg on each side (9)
(Other definitions for astraddle that I've seen before include "Across" , "How the jockey wants to be" , "with legs wide apart" , "With a leg on either side of (something)" .)
'with one leg on each side' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for astride that I've seen before include "On the back of" , "Not side saddle" , "With one leg on either side, as sitting on horse" , "With legs stretched apart - tirades (anag)" , "With legs on each side" .)