Be astride (8)
I believe the answer is:
straddle
'be astride' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for straddle that I've seen before include "Gymnastic exercise with a leg on either side of parallel bars" , "Stand with legs wide apart" , "Bestride" , "Extend over a certain area" , "Have one leg on each side of" .)