I am uppermost in their thoughts (8)
I believe the answer is:
egotists
'thoughts' is the definition.
Although both the answer and definition are plural nouns, I can't see how one could define the other.
'i am uppermost in their' is the wordplay.
I cannot really see how this works, but
'i' could be 'ego' ('I' in Latin) and 'ego' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for egotists that I've seen before include "They have an exaggerated sense of their own importance" , "The selfish" , "Conceited and self-centered people" , "Self-centred self-seekers" , "People with exaggerated notions of their own importance" .)