Most senior steed managed to get fifty (6)
I believe the answer is:
eldest
'most senior' is the definition.
(I've seen this before)
'steed managed to get fifty' is the wordplay.
'managed' indicates an anagram.
'to get' means one lot of letters goes inside another.
'fifty' becomes 'l' (roman numeral).
'steed' with letters rearranged gives 'edest'.
'edest' going around 'l' is 'ELDEST'.
(Other definitions for eldest that I've seen before include "Of greatest age" , "First in the order of birth" , "Most advanced in age, led set (anag.)" , "Most old" , "Most senior of a group" .)