Youngsters have caught the wrong bus (4)
I believe the answer is:
cubs
'youngsters' is the definition.
(cub can mean a baby or child)
'caught the wrong bus' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'the wrong' indicates an anagram.
'bus' anagrammed gives 'ubs'.
'c'+'ubs'='CUBS'
'have' is the link.
(Other definitions for cubs that I've seen before include "Young of certain animals" , "Little foxes" , "Young and inexperienced reporters" , "Junior Scouts" , "Young bears" .)