A pie's changed colour (5)
I believe the answer is:
sepia
'colour' is the definition.
(sepia is a kind of colour)
'a pie's changed' is the wordplay.
'changed' indicates an anagram.
'a'+'pies'='apies'
'apies' is an anagram of '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" .)