Alan's mad about the organ (5)
'about the organ' is the definition.
(the nose is an organ)
'alan's mad' is the wordplay.
'mad' indicates an anagram.
'alans' with letters rearranged gives 'NASAL'.
(Other definitions for nasal that I've seen before include "adenoids", "Rhinal", "Part of helmet", "Letter such as M or N", "pinched".)