Initially nice old boy, the French aristocrat (5)
I believe the answer is:
noble
'aristocrat' is the definition.
(I know that noble is a type of male aristocrat)
'initially nice old boy the french' is the wordplay.
'initially' indicates taking the first letters.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'the french' becomes 'le' ('the' in French).
The initial letter of 'nice' is 'n'.
'n'+'ob'+'le'='NOBLE'
(Other definitions for noble that I've seen before include "Dignified" , "Illustrious - of high birth - old gold coin" , "Illustrious - old money" , "Stately" , "gold coin, now obsolete" .)