Essentially cocky, Athos and Aramis developed allure (8)
I believe the answer is:
charisma
'allure' is the definition.
(I've seen this before)
'essentially cocky athos and aramis developed' is the wordplay.
'essentially cocky athos' becomes 'ch' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' says to put letters next to each other.
'developed' indicates anagramming the letters.
'aramis' is an anagram of 'arisma'.
'ch'+'arisma'='CHARISMA'
Can you help me to learn more?
(Other definitions for charisma that I've seen before include "Allure" , "divinely conferred talent?" , "Compelling charm" , "attractive quality" , "Amish car (anag.)" .)