Form of radiation discovered by Wilhelm Roentgen (1,3)
I believe the answer is:
x ray
'form of radiation' is the definition.
'X ray' can be an answer for 'radiation' (X ray is an example). I am unsure of the 'form of' bit.
'by wilhelm roentgen' is the wordplay.
I cannot really see how this works, but
'by' could be 'x' (multiplication sign) and 'x' is found in the answer.
'roentgen' could be 'r' (synonyms) and 'r' is found in the answer.
The remaining letters 'ay' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'discovered' acts as a link.
Can you help me to learn more?
(Other definitions for x ray that I've seen before include "Internal photo" , "penetrating sort of picture" , "Photograph" , "diagnostic aid" , "One after whisky" .)