Preoccupy (6)
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">preoccupy</span>' is the definition.<br/ ><span class="explanation-format__explanation">(obsessing is a kind of preoccupying)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for obsess that I've seen before include "Plague" , "Prey on" , "Persistently preoccupy" , "Preoccupy continually" , "Constantly worry (about something)" .)
(Other definitions for fixate that I've seen before include "Become absorbed" , "Obsess (on something)" .)