A fellow in baby clothes is an old French aristocrat/soldier (9)
I believe the answer is:
lafayette
'an old french aristocratsoldier' is the definition.
I can't tell whether this defines the answer.
'a fellow in baby clothes' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'in' means one lot of letters goes inside another.
'baby clothes' becomes 'layette' (I've seen this before).
'a'+'f'='af'
'af' placed within 'layette' is 'LAFAYETTE'.
'is' acts as a link.
Can you help me to learn more?
(Another definition for lafayette that I've seen is " Revolutionary general".)