Obsessed by (4)
I believe the answer is:
into
'obsessed by' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for into that I've seen before include "Keen on, colloquially" , "Showing enthusiasm for" , "Enthusiastic about" , "Taking an active and lively interest (in something)" , "Fascinated by" .)