Be cool kid, endlessly undisciplined (7)
I believe the answer is:
chillax
'be cool' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'kid endlessly undisciplined' is the wordplay.
'kid' becomes 'child' (synonyms).
'endlessly' means to remove the last letter.
'undisciplined' becomes 'lax' (I've seen this in another clue).
'child' with its last letter taken off is 'chil'.
'chil'+'lax'='CHILLAX'
Can you help me to learn more?
(Other definitions for chillax that I've seen before include "Calm down and take it easy (informal)" , "Rest from work" , "Calm down and relax (informal)" .)