Ever so clever youth, to some extent (4)
I believe the answer is:
very
'ever so' is the definition.
(I've seen this before)
'clever youth to some extent' is the wordplay.
'to some extent' indicates a hidden word.
'VERY' is hidden in the letters of 'clever youth'.
(Other definitions for very that I've seen before include "Extremely, in a high degree" , "Actual; in a high degree" , "light, perhaps" , "Frightfully" , "Extremely - light" .)