A pie's possibly brown (5)
I believe the answer is:
sepia
'brown' is the definition.
(sepia is a kind of brown)
'a pie's possibly' is the wordplay.
'possibly' indicates anagramming the letters.
'a'+'pies'='apies'
'apies' with letters rearranged gives 'SEPIA'.
(Other definitions for sepia that I've seen before include "Reddish-brown colour as in old photos" , "Sort of ink" , "Shade of brown, as in old photographs" , "Rich reddish-brown colour" , "Colourant" .)