Youngster finally taken by supporter on course (4)
I believe the answer is:
teen
'youngster' is the definition.
(teenager)
'finally taken by supporter on course' is the wordplay.
'finally' says to take the final letters.
'by' is a charade indicator (letters next to each other).
'supporter on course' becomes 'tee' (a tee supports a ball on a golf course).
The final letter of 'taken' is 'n'.
'n' put after 'tee' is 'TEEN'.
(Other definitions for teen that I've seen before include "Adolescent, then adult-to-be" , "Youngster over twelve" , "Adolescent (informal)" , "Juvenile" , "Youngster of a certain age" .)