Preoccupy, fixate (6)
I believe the answer is:
obsess
This clue is a double definition.
'preoccupy' is the first definition.
(obsessing is a kind of preoccupying)
'fixate' is the second definition.
(I've seen this in another clue)
(Other definitions for obsess that I've seen before include "Prey on" , "Control" , "Persistently preoccupy" , "Be constantly talking about" , "Preoccupy continually" .)