Obsessive type blocks inner demons (4)
I believe the answer is:
nerd
'obsessive type' is the definition.
(a nerd is someone obsessed with a particular topic)
'blocks inner demons' is the wordplay.
'blocks' indicates the answer is hidden within the clue (the answer plugs or blocks part of the clue).
'NERD' is hidden in the letters of 'inner demons'.
(Other definitions for nerd that I've seen before include "In slang, a foolish or uninteresting person" , "Foolish or annoying person" , "Boring, trivia-obsessed type" , "Anorak (informal)" , "one that's overly cerebral" .)