Little is taken in by dumb vacant nerd (5)
I believe the answer is:
dweeb
'nerd' is the definition.
(I've seen this before)
'little is taken in by dumb vacant' is the wordplay.
'little' becomes 'wee' ('wee' can be similar in meaning to 'little'**).
'is taken in by' is an insertion indicator.
'vacant' suggests removing the centre.
'dumb' with its middle taken out is 'db'.
'wee' placed within 'db' is 'DWEEB'.
(Other definitions for dweeb that I've seen before include "Antisocial guy" , "Insignificant student" , "Fool" , "Geek" , "Boringly studious student" .)