You're thankful Karen left while still young (8)
I believe the answer is:
youthful
'young' is the definition.
(similar in meaning)
'you're thankful karen left' is the wordplay.
'left' is a deletion indicator.
'youre'+'thankful'='yourethankful'
'yourethankful' with 'karen' taken away is 'YOUTHFUL'.
'while still' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for youthful that I've seen before include "Fresh and vigorous" , "Juvenile" , "Seeming young" , "Young in style" , "Sprightly" .)