Firstborn (6)
'firstborn' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for oldest that I've seen before include "Least recent" , "Most aged" , "Eldest" , "Most senior in age" , "First in time" .)
'firstborn' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for eldest that I've seen before include "Most advanced in age, led set (anag.)" , "Firstborn; led set (anag.)" , "Most senior of a group" , "Most senior (E, not O)" , "Most senior in age" .)