Fifty-one in surplus - yet he asked for more! (6)
I believe the answer is:
oliver
'he asked for more' is the definition.
(I've seen this before)
'fifty-one in surplus' is the wordplay.
'fifty one' becomes 'LI' (Roman numerals).
'in' is an insertion indicator.
'surplus' becomes 'over' (I've seen this before).
'li' going within 'over' is 'OLIVER'.
'yet' is the link.
(Other definitions for oliver that I've seen before include "Roland companion; Bath biscuit" , "Dickens-based musical" , "Musical from Dickens novel" , "Character in Dickens" , "Master Twist" .)