Horse, note, developed a brain (7)
I believe the answer is:
arabian
'horse' is the definition.
(I know that Arabian is a type of mount)
'note developed a brain' is the wordplay.
'note' becomes 'a' (musical note).
'developed' is an anagram indicator.
'a'+'brain'='abrain'
'abrain' anagrammed gives 'rabian'.
'a'+'rabian'='ARABIAN'
(Other definitions for arabian that I've seen before include "from Bahrain, perhaps" , "Saudi, perhaps" , "Darley, for one" , "Breed of horse" , "Sea in the northwestern part of the Indian Ocean" .)