Reddish-brown start to autumn, in two different ways (5)
I believe the answer is:
sepia
'reddish-brown' is the definition.
(I've seen this before)
'start to autumn in two different ways' is the wordplay.
'start to autumn' becomes 'Sep I' (resembles Sep a, ie the ast September).
'in two different ways' becomes 'a' (I can't justify this - if you can you should believe this answer much more).
'sepi'+'a'='SEPIA'
(Other definitions for sepia that I've seen before include "Sort of ink" , "Reddish-brown colour as in old photos" , "Reddish brown -- ie sap (anag)" , "Former photographic pigment" , "Brown tone associated with old photographs" .)