Great rap moves involving boy, soon an outcast (7,3,5)
I believe the answer is:
persona non grata
'outcast' is the definition.
(thesaurus)
'great rap moves involving boy soon an' is the wordplay.
I cannot quite see how this works, but
'great' could be 'gr' and 'gr' is found in the answer.
'boy' could be 'nat' and 'nat' is found in the remaining letters.
'an' could be 'per' (synonymous as in 'once an hour', 'once per hour') and 'per' is found within the answer.
This explanation may well be incorrect...
(Other definitions for persona non grata that I've seen before include "One banned" , "Undesirable" .)