"He's over fifty", I put in (6)
I believe the answer is:
oliver
'he's' is the definition.
'over fifty i put in' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'put in' means one lot of letters goes inside another.
'l'+'i'='li'
'over' enclosing 'li' is 'OLIVER'.
(Other definitions for oliver that I've seen before include "Dickens boy named Twist" , "Musical from Dickens novel" , "Dickens boy who asked for more, and a musical comedy" , "Musical based on the story of Dickens boy" , "One taking pride in food" .)