An egghead, seemingly encamped, is dogmatic (5)
I believe the answer is:
tenet
'an egghead' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for tenet that I've seen before include "Matter of belief" , "A palindromic doctrine or creed" , "Dogma or principle" , "Belief held as true" , "A religious doctrine proclaimed as true without proof" .)