Article on a brain disturbance in a horse (7)
I believe the answer is:
arabian
'horse' is the definition.
(I know that Arabian is a horse breed)
'article on a brain disturbance' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'on' indicates putting letters inside (as in clothing 'on' a person).
'disturbance' indicates an anagram.
'brain' is an anagram of 'rabin'.
'a' placed inside 'rabin' is 'rabian'.
'a'+'rabian'='ARABIAN'
'in a' is the link.
(Other definitions for arabian that I've seen before include "These Nights provide fairy tales" , "Middle Easterner; horse" , "Of a Middle East area" , "Like Saudi, Yemeni, etc" , "Darley, for one" .)