Rational article in south-east (4)
I believe the answer is:
sane
'rational' is the definition.
(similar in meaning)
'article in south-east' is the wordplay.
'article' becomes 'an' (indefinite article).
'in' indicates putting letters inside.
'south east' becomes 'SE' (abbreviation).
'an' going into 'se' is 'SANE'.
(Other definitions for sane that I've seen before include "Sound in mind" , "Mentally sound" , "Compos mentis" , "Healthy in mind" , "In full possession of mental faculties" .)