Joy's nightmares! (6)
I believe the answer is:
dreams
'joy's nightmares' is the definition.
The answer and definition can be both related to thinking as well as being plural nouns.
Perhaps they are linked in a way I don't understand?
This is the entire clue.
Can you help me to learn more?
(Other definitions for dreams that I've seen before include "Imaginings" , "'We are such stuff / As dreams are made on' (The Tempest)" , "Fantasies; fantasises" , "Fantasy ambitions" , "Reveries or cherished aspirations" .)