Rock in more expansive bit of space always losing velocity (8)
I believe the answer is:
asteroid
'rock' is the definition.
(an asteroid is a rock in space)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for asteroid that I've seen before include "Small body orbiting sun" , "Bit of rock that moves around the heavens" , "Rocky body in orbit" , "Small planet orbiting the Sun" , "Small planet revolving round the sun" .)