Very serious additional discussion tails off (6)
I believe the answer is:
mortal
This might be a double definition.
'very serious additional discussion tails' is the first definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'off' is the second definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?
Can you help me to learn more?
(Other definitions for mortal that I've seen before include "Human - transitory" , "Liable to cause death" , "Very drunk < person" , "Fatal" , "Sound a wound is deadly" .)