Boy’s fast time — when failing to finish (3)
'boy's' is the definition.
(I've seen this before)
'fast time when failing to finish' is the wordplay.
'fast time' becomes 'lent' (I've seen this before).
'when failing to finish' means to remove the last letter.
'lent' with its last letter taken off is 'LEN''.
(Other definitions for len that I've seen before include "Sir - - Hutton", "- - Hutton, cricketer", "Boy", "-- Deighton, author", "Leonard (dim.)".)