Boy’s fast, but failing to finish (3)
I believe the answer is:
len
'boy's' is the definition.
(boy's name)
'fast but failing to finish' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'but failing to finish' means to remove the last letter.
'lent' with its final letter removed is 'LEN''.
(Other definitions for len that I've seen before include "Sir - - Hutton" , "- - Hutton, cricketer" , "Boy" , "Leonard (dim.)" , "-- Deighton, author" .)