Youngster of innate enthusiasm (4)
I believe the answer is:
teen
'youngster' is the definition.
(I've seen this before)
'of innate enthusiasm' is the wordplay.
'of' says the answer is hidden in the clue.
'TEEN' can be found hidden inside 'innate enthusiasm'.
(Other definitions for teen that I've seen before include "Of 13 to 19 year olds" , "Youngster of a certain age" , "Young person" , "Youth" , "Youngster over twelve" .)