One among back copies reveals tone (5)
I believe the answer is:
sepia
'tone' is the definition.
'one among back copies' is the wordplay.
'one' becomes 'i' (Roman numeral).
'among' means one lot of letters goes inside another (inserted letters are placed among others).
'back' shows that the letters should be reversed in order.
'copies' becomes 'apes' ('ape' can be a synonym of 'copy').
'apes' back-to-front is 'sepa'.
'i' going within 'sepa' is 'SEPIA'.
'reveals' acts as a link.
(Other definitions for sepia that I've seen before include "Brown pigment produced from cuttlefish ink" , "Reddish-brown colour as in old photos" , "Colour of old photographs" , "Dark reddish-brown pigment" , "Reddish-brown colour - easy as pie" .)