Rocking Le Bon magnificent (5)
I believe the answer is:
noble
'magnificent' is the definition.
(I've seen this before)
'rocking le bon' is the wordplay.
'rocking' is an anagram indicator.
'le'+'bon'='lebon'
'lebon' is an anagram of 'NOBLE'.
(Other definitions for noble that I've seen before include "old British coin" , "Blue-blooded" , "fine" , "Lord" , "Illustrious - of high birth - old gold coin" .)