Those pictures put behind the glass, are identical (6,6)
I believe the answer is:
mirror images
'are identical' is the definition.
I can't tell whether this definition defines the answer.
'those pictures put behind the glass' is the wordplay.
I cannot quite understand how this works, but
'pictures' could be 'images' ('image' can be a synonym of 'picture') and 'images' is located in the answer.
'glass' could be 'mirror' (a looking glass is a mirror) and 'mirror' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for mirror images that I've seen is " results of reflection".)