One-time citizen’s infant boy consuming half of capital (10)
I believe the answer is:
babylonian
'one-time citizen's' is the definition.
(citizen of the historical city of Babylon)
'infant boy consuming half of capital' is the wordplay.
'infant' becomes 'baby' (synonyms).
'boy' becomes 'Ian' (man's name).
'consuming' means one lot of letters goes inside another.
'half of capital' becomes 'lon' (I've seen this in other clues).
'baby'+'ian'='babyian'
'babyian' enclosing 'lon' is 'BABYLONIAN'.
(Other definitions for babylonian that I've seen before include "Old citizen" , "From the old city" .)