It may be produced by carbon monoxide to a degree (4)
I believe the answer is:
coma
'it may be' is the definition.
The definition suggests a singular noun which matches the answer.
'by carbon monoxide to a degree' is the wordplay.
'by' is a charade indicator (letters next to each other).
'carbon monoxide' becomes 'CO' (chemical formula for carbon monoxide).
'to a degree' becomes 'ma' (Master of Arts is a type of degree).
'co' put next to 'ma' is 'COMA'.
'produced' acts as a link.
Can you help me to learn more?
(Other definitions for coma that I've seen before include "Deep sleep" , "Long period of unconsciousness" , "State of unconsciousness caused by illness" , "Stupor" , "Unconsciousness from which person cannot be aroused" .)