He rapidly scored a second, with variable skill (6)
I believe the answer is:
mozart
'he rapidly scored' is the definition.
(prolific composer of music - written on a score)
'a second with variable skill' is the wordplay.
'a second' becomes 'mo' (both are short periods of time).
'with' says to put letters next to each other.
'variable' becomes 'z' (algebra).
'skill' becomes 'art' (both can mean a talent).
'mo'+'z'+'art'='MOZART'
(Other definitions for mozart that I've seen before include "Wolfgang Amadeus . . . . . . , great Austrian composer" , "'Cosi fan tutte' composer" , "Austrian composer and child prodigy" , "Eine kleine Nachtmusik composer" , "Scorer" .)