Initially, rookie or youth, associate with great style (7)
I believe the answer is:
royally
'with great style' is the definition.
(royally can mean to an extreme degree)
'initially rookie or youth associate' is the wordplay.
'initially' indicates taking the first letters.
'associate' becomes 'ally' (ally is a kind of associate).
The first letters of 'rookie or youth' is 'roy'.
'roy'+'ally'='ROYALLY'
(Other definitions for royally that I've seen before include "With magnificent style" , "in splendid fashion" , "Utterly" , "Grandly" .)