Saw a youngster at heart, on vacation! (4)
I believe the answer is:
away
'on vacation' is the definition.
(I've seen this in another clue)
'saw a youngster at heart' is the wordplay.
'at heart' indicates a hidden word (letters hidden in the heart of the clue).
'AWAY' is hidden within 'saw a youngster'.
(Other definitions for away that I've seen before include "Away in the distance" , "travelling" , "Distant or not present" , "Out of town" , "See 1" .)