A piano expert is fast (5)
I believe the answer is:
apace
'is fast' is the definition.
(apace can mean quickly)
'a piano expert' is the wordplay.
'piano' becomes 'p' (musical abbreviation).
'expert' becomes 'ace' (ace can informally mean very good).
'a'+'p'+'ace'='APACE'
(Other definitions for apace that I've seen before include "At a rapid rate" , "at high speed" , "Swiftly, quickly" , "(Move on) swiftly" , "Quick" .)