Dominate worst tour (8)
I believe the answer is:
bestride
'dominate' is the definition.
(thesaurus)
'worst tour' is the wordplay.
'worst' becomes 'best' (both can mean to beat someone).
'tour' becomes 'ride'.
'best'+'ride'='BESTRIDE'
(Other definitions for bestride that I've seen before include "get a legover?" , "Stand over protectively" , "Mount" , "Get across" , "Protect" .)