Broken-down steed about fifty - most advanced in age (6)
I believe the answer is:
'most advanced in age' is the definition.
(I've seen this before)
'broken-down steed about fifty' is the wordplay.
'broken down' is an anagram indicator.
'about' means one lot of letters goes inside another.
'fifty' becomes 'L' (Roman numeral).
'steed' with letters rearranged gives 'edest'.
'edest' placed around 'l' is 'ELDEST'.
(Other definitions for eldest that I've seen before include "Most advanced in age, led set (anag.)", "Such as Cain, say", "Of greatest age", "Most senior (E, not O)", "Most senior of two".)