Alan's mad about the organ (5)
I believe the answer is:
nasal
'about the organ' is the definition.
(the nose is an organ)
'alan's mad' is the wordplay.
'mad' indicates anagramming the letters.
'alans' is an anagram of 'NASAL'.
(Other definitions for nasal that I've seen before include "of central feature" , "Beak's" , "nosy?" , "Concerning the nose" , "like the sound from an organ" .)