Youth infinite, endless to some extent? (4)
I believe the answer is:
teen
'youth' is the definition.
(I've seen this before)
'infinite endless to some extent?' is the wordplay.
'to some extent?' says the answer is hidden in the clue (I've seen 'to some extent' mean this).
'TEEN' can be found hidden inside 'infinite endless'.
(Other definitions for teen that I've seen before include "Youngster of a certain age" , "Youth" , "Young person" , "Adolescent (informal)" , "Adolescent, then adult-to-be" .)