Dominate (8)
'dominate' is the definition.
(I know that overbear is a more specific form of dominate)
This is all the clue.
(Other definitions for overbear that I've seen before include "Dominate" , "Be oppressive" , "Oppress by emotional/physical pressure" , "Bully" , "Act in a particular way" .)
(Other definitions for tower that I've seen before include "Tall building - loom (over)" , "Monument" , "Tall structure" , "Campanile, spire etc" , "tractor, perhaps" .)