Whizz over finale of Bach, needing one piece for bar (8)
I believe the answer is:
prohibit
'for bar' is the definition.
(I've seen this in another clue)
'whizz over finale of bach needing one piece' is the wordplay.
'whizz over' becomes 'pro' (both can mean someone skilled at something. I am not sure about the 'over' bit.).
'finale of' indicates one should take the final letters.
'needing' means one lot of letters go next to another.
'one' becomes 'I' (Roman numeral).
'piece' becomes 'bit' (bit is a kind of piece).
The last letter of 'bach' is 'h'.
'pro'+'h'+'i'+'bit'='PROHIBIT'
(Other definitions for prohibit that I've seen before include "Bar" , "Disallow" , "Stop" , "Forbid, ban" , "Proscribe, forbid" .)