Prone, racing, to go to topless (5)
I believe the answer is:
lying
'prone' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for lying that I've seen before include "Flat-out" , "Deceit" , "Extending" , "production of pork pies?" , "Spread out" .)