Alan's mad about organ (5)
I believe the answer is:
nasal
'about organ' is the definition.
(relating to the nose)
'alan's mad' is the wordplay.
'mad' indicates anagramming the letters.
'alans' with letters rearranged gives 'NASAL'.
(Other definitions for nasal that I've seen before include "To do with nose, say sound" , "front piece of helmet" , "Nosey" , "Hooters" , "Like sounds of m, n, ng" .)