Gas lit behind unit for a physicist (5)
I believe the answer is:
radon
'gas' is the definition.
(radon is a gas)
'lit behind unit for a physicist' is the wordplay.
I cannot really understand how this works, but
'lit' could be 'on' (eg a light that is switched on) and 'on' is present in the answer.
'unit' could be 'rad' (rad is a kind of unit) and 'rad' is present in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for radon that I've seen before include "Naturally occurring inert gas" , "Radioactive gas, linked to granite" , "hazardous substance" , "Inert gas that occurs naturally" , "Gaseous element, Rn" .)