Thought ageist characters should be excluded from negotiations (6)
I believe the answer is:
notion
'thought' is the definition.
(I know that notion is a type of thought)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for notion that I've seen before include "Whimsical idea or small sewing article" , "clue" , "Idea or fancy" , "Vague or whimsical idea" , "A vague and possibly incorrect opinion" .)