Even unacceptable at top speed (4,3)
I believe the answer is:
flat out
'at top speed' is the definition.
(working flat out is working fast)
'even unacceptable' is the wordplay.
'even' becomes 'flat' (similar in meaning).
'unacceptable' becomes 'out' (I've seen this before).
'flat'+'out'='FLAT OUT'
(Other definitions for flat out that I've seen before include "At top speed; intense (effort)" , "KO'd" , "As fast as possible" , "Bluntly" , "Lying on floor" .)