Is important and one doesn’t leave out of the reckoning (6)
I believe the answer is:
counts
'is' is the definition.
The answer and definition can be both to do with fundamental existence as well as being verbs in their -s form.
Perhaps there's a link between them I don't understand?
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for counts that I've seen before include "Determines the number of" , "Enumerates like minor aristocracy" , "Numbers" .)