Layer hedging French king's horse (6)
I believe the answer is:
heroin
'horse' is the definition.
(informal term for heroin)
'layer hedging french king's' is the wordplay.
'layer' becomes 'hen' (hens lay eggs).
'hedging' means one lot of letters goes inside another (some letters hedge or border others).
'french king' becomes 'roi' ('king' in French).
'hen' going around 'roi' is 'HEROIN'.
(Other definitions for heroin that I've seen before include "Highly addictive drug madefrom morphine" , "Class A drug" , "Highly addictive morphine derivative" , "junk" , "Hard drug" .)