A performance-enhancing drug for a flier (8)
I believe the answer is:
asteroid
'flier' is the definition.
'a performance-enhancing drug for a' is the wordplay.
'a performance-enhancing drug' becomes 'steroid'.
'for' says to put letters next to each other (I've seen this in other clues).
'steroid' put after 'a' is 'ASTEROID'.
(Other definitions for asteroid that I've seen before include "Small celestial body moving around the sun" , "could be dangerous for astronauts" , "Small rock orbiting the sun" , "looking like a star?" , "Piece of rock that moves around the heavens" .)