Broken-down steed about fifty - most advanced in age (6)
I believe the answer is:
eldest
'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 aged" , "Most senior in age" , "First in the order of birth" , "Of greatest age" , "Most old" .)