Going at full speed - or lying prone on the ground (4,3)
I believe the answer is:
flat out
'going at full speed or lying' is the definition.
I can't tell whether this definition defines the answer.
'prone on the ground' is the wordplay.
'prone' becomes 'flat' (I've seen this in another clue).
'on the ground' becomes 'out' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'flat'+'out'='FLAT OUT'
Can you help me to learn more?
(Other definitions for flat out that I've seen before include "Ready to drop" , "Exhausted - at full speed" , "full tilt?" , "At top speed - totally exhausted" , "At top speed or effort - being horizontal?" .)