Youngster’s support on course, needed at the start (4)
I believe the answer is:
teen
'youngster's' is the definition.
(I've seen this in another clue)
'support on course needed at the start' is the wordplay.
'support on course' becomes 'tee' (golf tee on a golf course).
'at the start' indicates taking the first letters.
The first letter of 'needed' is 'n'.
'tee'+'n'='TEEN'
(Other definitions for teen that I've seen before include "Youngster over twelve" , "Juvenile" , "Adolescent (informal)" , "Adolescent, then adult-to-be" , "Young person" .)