Optical instrument used to observe transparent material (8)
I believe the answer is:
spyglass
'optical instrument' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'to observe transparent material' is the wordplay.
'to observe' becomes 'spy' (I am not sure about this - if you are sure you should believe this answer much more).
'transparent material' becomes 'glass' (I've seen this before).
'spy'+'glass'='SPYGLASS'
'used' acts as a link.
Can you help me to learn more?
(Other definitions for spyglass that I've seen before include "lookout's aid" , "he can see through it!" , "See-through article" , "Aid to pirate, perhaps" , "Small refracting telescope" .)