As fast as possible (4,3)
I believe the answer is:
flat out
'as fast as possible' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for flat out that I've seen before include "Ready to drop" , "Exhausted - at full speed" , "Using every effort" , "At top speed or effort - being horizontal?" , "Bluntly" .)