Horse is major character in audiobook (6)
I believe the answer is:
heroin
'horse' is the definition.
(informal term for heroin)
'major character in audiobook' is the wordplay.
I cannot really see how this works, but
'character' could be 'hero' (hero is a kind of character) and 'hero' is found in the answer.
'in' is within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for heroin that I've seen before include "Highly addictive morphine derivative" , "junk" , "Highly addictive drug madefrom morphine" , "Narcotic powder derived from morphine" , "Highly addictive narcotic" .)