Most senior (6)
'most senior' is the definition.
('old' is associated in meaning with 'senior')
This is all the clue.
(Other definitions for oldest that I've seen before include "First in time" , "Least young" , "Most aged" , "Least recent" , "Most ancient (O, not E)" .)
'most senior' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for eldest that I've seen before include "Most advanced in years" , "Such as Cain, say" , "Firstborn; led set (anag.)" , "Most senior in age" , "First in the order of birth" .)