Most aristocratic old boy left in retreat (7)
I believe the answer is:
noblest
'most aristocratic' is the definition.
('noble' can be similar in meaning to 'aristocratic')
'old boy left in retreat' is the wordplay.
'old boy' becomes 'ob'.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'retreat' becomes 'nest' (nest is a kind of retreat).
'ob'+'l'='obl'
'obl' placed within 'nest' is 'NOBLEST'.
(Other definitions for noblest that I've seen before include "Most refined" , "Highest-ranking" , "Most magnificent" , "most generous" , "Most dignified" .)