Youthful Achilles had no seal on his plate. (8)
I believe the answer is:
childish
'youthful' is the definition.
Both the answer and definition are adjectives. Maybe you can see a link between them that I can't see?
'achilles had no seal on his plate' is the wordplay.
'had no' is a deletion indicator.
'on his plate' becomes 'dish' (I can't justify this - if you can you should give a lot more credence to this answer).
'achilles' with 'seal' taken away is 'chil'.
'chil'+'dish'='CHILDISH'
Can you help me to learn more?
(Other definitions for childish that I've seen before include "Juvenile" , "Puerile" , "Silly, immature" .)