Abusive, possibly rash after a hasty start (5)
I believe the answer is:
harsh
'abusive' is the definition.
(thesaurus)
'possibly rash after a hasty start' is the wordplay.
'possibly' indicates anagramming the letters.
'after' is a charade indicator (letters next to each other).
'start' indicates taking the first letters.
The first letter of 'hasty' is 'h'.
'rash' anagrammed gives 'hars'.
'hars'+'h'='HARSH'
(Other definitions for harsh that I've seen before include "Strident" , "Grating; severe" , "Punitive" , "Jarring" , "Abrasive, rough or unkind" .)