Youth spotted this problem (4)
I believe the answer is:
acne
'this problem' is the definition.
(I have seen 'Youthful problem' mean 'acne' so perhaps 'problem' could also mean 'acne')
I cannot understand how the rest of the clue works.
(Other definitions for acne that I've seen before include "Spotty coverage" , "Adolescent spots" , "Pimply skin condition of adolescence" , "Skin disorder" , "sort of disease" .)