American author adopting crazy hair treatment (6)
I believe the answer is:
pomade
'hair treatment' is the definition.
(I've seen this in another clue)
'american author adopting crazy' is the wordplay.
'american author' becomes 'Poe' (Edgar Allan Poe).
'adopting' is an insertion indicator (inserted letters are taken on or adopted).
'crazy' becomes 'mad' (synonyms).
'poe' going around 'mad' is 'POMADE'.
(Other definitions for pomade that I've seen before include "hair treatment" , "capital application" , "Scented ointment for the scalp" , "Scented hair dressing" , "Oily dressing for hair" .)