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' says to put letters next to each other.
'ass' becomes 'fool' (both can mean a stupid person).
'hardy' after 'fool' is 'FOOLHARDY'.
(Other definitions for foolhardy that I've seen before include "Reckless" , "Impulsive" , "Hot-headed" , "Bold" , "Rash - incautious" .)