Whiskey to assist youngster (5)
I believe the answer is:
whelp
'youngster' is the definition.
(I've seen this before)
'whiskey to assist' is the wordplay.
'whiskey' becomes 'W' (alpha bravo charlie delta ...).
'to assist' becomes 'help' ('help' can be a synonym of 'assist').
'w'+'help'='WHELP'
(Other definitions for whelp that I've seen before include "impudent young lad" , "Puppy- young wolf" , "Give birth to (a puppy)" , "Puppy, cub" , "youngster" .)