Children half-recalled obsessive type (4)
I believe the answer is:
nerd
'obsessive type' is the definition.
(a nerd is someone obsessed with a particular topic)
'children half-recalled' is the wordplay.
'half' means to take half the letters.
'recalled' shows that the letters should be reversed in order (to recall can mean to bring something back).
'children' with half the letters taken is 'dren'.
'dren' in reverse letter order is 'NERD'.
(Other definitions for nerd that I've seen before include "Anorak (informal)" , "Obsessive," , "Geek, anorak (sl.)" , "One lacking social skills" , "Boring person" .)