Back copies one acquired are a brownish colour (5)
I believe the answer is:
sepia
'a brownish colour' is the definition.
(I've seen this before)
'back copies one acquired' is the wordplay.
'back' is a reversal indicator.
'copies' becomes 'apes' ('ape' can be a synonym of 'copy').
'one' becomes 'i' (Roman numeral).
'acquired' means one lot of letters goes inside another (inserted letters are acquired by another word).
'apes' enclosing 'i' is 'aipes'.
'aipes' back-to-front is 'SEPIA'.
'are' is the link.
(Other definitions for sepia that I've seen before include "Brown pigment produced from cuttlefish ink" , "Reddish-brown colour as in old photos" , "A greyish brown" , "tone of pictures?" , "Brown colour of old photo" .)