The Utopian has his own collection of notions (8)
I believe the answer is:
This could be a double definition.
'the utopian' is the first definition.
(I've seen this in another clue)
'his own collection of notions' is the second definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'has' is the link.
Can you help me to learn more?
(Other definitions for idealist that I've seen before include "Person with noble aspirations, perhaps unrealistic" , "An optimist or romantic" , "One sees things as they should be rather than how they are" , "Visionary" , "One who aspires to a perfect world" .)