Rational article in south-east (4)
I believe the answer is:
sane
'rational' is the definition.
(associated in meaning)
'article in south-east' is the wordplay.
'article' becomes 'an' (indefinite article).
'in' means one lot of letters goes inside another.
'south east' becomes 'SE' (abbreviation).
'an' put within '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" .)