A pie's turning brown (5)
I believe the answer is:
sepia
'brown' is the definition.
(I know that sepia is a type of brown)
'a pie's turning' is the wordplay.
'turning' indicates anagramming the letters.
'a'+'pies'='apies'
'apies' with letters rearranged gives 'SEPIA'.
(Other definitions for sepia that I've seen before include "Brown colour of old photo" , "Shade of brown, as in old photographs" , "Old drawing" , "Sort of drawing" , "Denizen of deep" .)