Most senior steed managed to get left (6)
I believe the answer is:
eldest
'most senior' is the definition.
(I've seen this in another clue)
'steed managed to get left' is the wordplay.
'managed' is an anagram indicator.
'to get' indicates putting letters inside.
'left' becomes 'l' (common abbreviation).
'steed' with letters rearranged gives 'edest'.
'edest' enclosing 'l' is 'ELDEST'.
(Other definitions for eldest that I've seen before include "First to be born" , "Firstborn; led set (anag.)" , "Most senior of two" , "Most aged" , "heir?" .)