Initially fast, then flat out? (6)
I believe the answer is:
flying
'out?' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe you can see a link between them that I don't see?
'initially fast then flat' is the wordplay.
'initially' indicates taking the first letters.
'then' is a charade indicator (letters next to each other).
'flat' becomes 'lying' (I've seen this before).
The initial letter of 'fast' is 'f'.
'f'+'lying'='FLYING'
(Other definitions for flying that I've seen before include "Airborne" , "(Of time) passing swiftly" , "(Of flag) displayed on a flagpole" , "Moving fast as in the air" , "- Boat; - bomb; - fox" .)