Ranking (7)
'ranking' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for senior that I've seen before include "Elder person - higher in rank" , "Elder (of two)" , "High-ranking" , "'Older, superior (6)'" , "More advanced in years or authority" .)
'ranking' is the definition.
('grade' can be a synonym of 'rank')
This is the entire clue.
(Another definition for grading that I've seen is " Ranking in order".)