Most senior (6)
'most senior' is the definition.
('old' can be similar in meaning to 'senior')
This is the entire 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 before)
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" .)