An organ for music (3)
I believe the answer is:
ear
This clue might be a double definition.
'an organ' is the first definition.
(ear is a kind of organ)
'music' is the second definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'for' acts as a link.
Can you help me to learn more?
(Other definitions for ear that I've seen before include "A corny bit" , "Fruiting head of corn" , "Speculator ignoring British" , "Aural organ" , "Organ inside heart" .)