Summit held in Dover, Texas (6)
I believe the answer is:
vertex
'summit' is the definition.
(both can mean something's highest point)
'held in dover texas' is the wordplay.
'held in' indicates the answer is hidden within the clue.
'VERTEX' can be found hidden inside 'dover texas'.
(Other definitions for vertex that I've seen before include "Crown, apex" , "Top, apex" , "Corner (of triangle)" , "Summit" , "where sides meet" .)