Nullify north-east entrance (6)
I believe the answer is:
negate
'nullify' is the definition.
(I know that nullify can be written as negate)
'north-east entrance' is the wordplay.
'north east' becomes 'ne'.
'entrance' becomes 'gate' (type of entrance).
'ne'+'gate'='NEGATE'
(Other definitions for negate that I've seen before include "Cancel; invalidate" , "Show to be false" , "Contradict" , "Nullify - deny the existence of" , "Cause to have no effect" .)