Mean to assert one's seniority (7)
I believe the answer is:
average
'mean' is the definition.
(I know that mean can be written as average)
'assert one's seniority' is the wordplay.
I cannot quite see how this works, but
'assert' could be 'aver' (synonyms) and 'aver' is present in the answer.
'seniority' could be 'age' (I've seen this in other clues) and 'age' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for average that I've seen before include "It's common" , "Normal in arithmetic way" , "Mean, median or middling" , "just like any other" , "Run of the mill" .)