Unknown fish bones may appear in this (1-3)
I believe the answer is:
x-ray
'bones may appear in this' is the definition.
The definition suggests a singular noun which matches the answer.
'unknown fish' is the wordplay.
'unknown' becomes 'x' (algebra).
'fish' becomes 'ray' (ray is a kind of fish).
'x'+'ray'='X-RAY'
Can you help me to learn more?
(Other definitions for x-ray that I've seen before include "Radiogram" , "Electromagnetic photograph" , "Medical photo" , "Internal "photo"" , "Radiation photograph" .)