A pie's changed colour (5)
I believe the answer is:
sepia
'colour' is the definition.
(I know that sepia is a colour)
'a pie's changed' is the wordplay.
'changed' is an anagram indicator.
'a'+'pies'='apies'
'apies' anagrammed gives 'SEPIA'.
(Other definitions for sepia that I've seen before include "Colourant" , "Shade of brown, as in old photographs" , "Rich reddish-brown colour" , "Brown tint" , "Dark reddish-brown pigment" .)