Perform like Eminem, casually fast (7)
I believe the answer is:
rapidly
'fast' is the definition.
(synonyms)
'perform like eminem casually' is the wordplay.
'perform like eminem' becomes 'rap' (Eminem is a rapper).
'casually' becomes 'idly' (I've seen this in another clue).
'rap'+'idly'='RAPIDLY'
(Other definitions for rapidly that I've seen before include "Fast" , "With great speed" , "Speedily" , "At a great rate" , "right off the bat" .)