Superlatively youthful (8)
I believe the answer is:
youngest
'superlatively youthful' is the definition.
I can't tell whether this defines the answer.
'youthful' is the wordplay.
I cannot really understand how this works, but
'youthful' could be 'young' (similar in meaning) and 'young' is found in the answer.
The remaining letters 'est' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for youngest that I've seen before include "Least old" , "Most junior" , "least experienced" , "last born" , "Ath the earliest stage of development" .)