Impetuous author following ass (9)
I believe the answer is:
foolhardy
'impetuous' is the definition.
(I've seen this in another clue)
'author following ass' is the wordplay.
'author' becomes 'hardy' (Thomas Hardy).
'following' means one lot of letters go next to another.
'ass' becomes 'fool' (both can mean a stupid person).
'hardy' put after 'fool' is 'FOOLHARDY'.
(Other definitions for foolhardy that I've seen before include "Reckless" , "Impulsive" , "Hot-headed" , "Bold" , "Rash - incautious" .)