Start of Irish National led by champion horse (6)
I believe the answer is:
heroin
'horse' is the definition.
(informal term for heroin)
'start of irish national led by champion' is the wordplay.
'start of' says to take the initial letters.
'led by' means one lot of letters go next to another (some letters go in front of others).
'champion' becomes 'hero' (synonyms).
The initial letters of 'irish national' is 'in'.
'in' after 'hero' is 'HEROIN'.
(Other definitions for heroin that I've seen before include "Highly addictive morphine derivative" , "Hard drug" , "Class A drug" , "Type of drug" , "junk" .)