Extremely serious letter indicative of owning top location (8)
I believe the answer is:
severest
'indicative of owning top location' is the definition.
I can't judge whether this definition defines the answer.
'extremely serious letter' is the wordplay.
I cannot really understand how this works, but
'serious' could be 'severe' (synonyms) and 'severe' is located in the answer.
'letter' could be 't' and 't' is located in the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for severest that I've seen before include "Most strict and harsh" , "Most extreme or difficult" , "Most dangerous" , "Most harsh and punitive" .)