Alan's mad about an organ (5)
I believe the answer is:
nasal
'an organ' is the definition.
The definition and answer can be both body parts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'alan's mad' is the wordplay.
'mad' is an anagram indicator.
'alans' anagrammed gives 'NASAL'.
'about' is the link.
Can you help me to learn more?
(Other definitions for nasal that I've seen before include "Sounding affected by cold" , "Part of helmet" , "description of Alain's vowels" , "Like some spray" , "Nosey" .)