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