Most senior steed managed to get left (6)
I believe the answer is:
eldest
'most senior' is the definition.
(I've seen this before)
'steed managed to get left' is the wordplay.
'managed' is an anagram indicator.
'to get' means one lot of letters goes inside another.
'left' becomes 'l' (common abbreviation).
'steed' anagrammed gives 'edest'.
'edest' placed around '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?" .)