Convert to Presley, rocking (9)
I believe the answer is:
proselyte
'convert' is the definition.
(term for a religious convert)
'to presley rocking' is the wordplay.
'rocking' is an anagram indicator.
'to'+'presley'='topresley'
'topresley' with letters rearranged gives 'PROSELYTE'.
(Other definitions for proselyte that I've seen before include "recruit" , "Convert to a religion, particularly Jewish" , "Religious convert" , "One now thinks differently" .)