Fixate, preoccupy (6)
I believe the answer is:
obsess
I believe this is a double definition.
'fixate' is the first definition.
(I've seen this in another clue)
'preoccupy' is the second definition.
(I know that obsess is a more specific form of preoccupy)
(Other definitions for obsess that I've seen before include "Occupy the thoughts persistently" , "Control" , "Preoccupy completely" , "Bosses (anag.)" , "brood" .)