Oddly, liar swears they're using a device emitting electromagnetic radiation (5)
I believe the answer is:
laser
'using a device emitting electromagnetic radiation' is the definition.
I can't judge whether this defines the answer.
'oddly liar swears' is the wordplay.
'oddly' means one should take alternating letters (letters in odd-numbered positions).
'liar'+'swears'='liarswears'
The alternating letters of 'liarswears' are 'LASER'.
'they're' is the link.
Can you help me to learn more?
(Other definitions for laser that I've seen before include "Device that produces an intense beam of light" , "Intense and concentrated beam of light" , "scientific instrument" , "Optical device producing an intense beam of light" , "Device producing thin beam of light" .)