A youthful Barker's serve wide at first (5)
I believe the answer is:
whelp
'a youthful barker's' is the definition.
(old-fashioned term for a puppy)
'serve wide at first' is the wordplay.
'serve' becomes 'help' (helping is a kind of serving).
'wide' becomes 'w' (abbreviation e.g. in cricket).
'at first' is a charade indicator (letters next to each other).
'help' after 'w' is 'WHELP'.
(Other definitions for whelp that I've seen before include "Puppy, cub" , "Perform deliveries without assistance" , "Young mammal" , "youngster" , "Produce issue" .)