Become fixated (6)
I believe the answer is:
obsess
'become fixated' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for obsess that I've seen before include "Persistently preoccupy" , "Constantly worry (about something)" , "Be constantly preoccupied" , "Bosses (anag.)" , "Haunt - preoccupy" .)