Impulsive grass crossing good man (5)
I believe the answer is:
hasty
'impulsive' is the definition.
(thesaurus)
'grass crossing good man' is the wordplay.
'grass' becomes 'hay' (I have seen 'Mown grass ' mean 'hay' so perhaps 'grass' could also mean 'hay'**).
'crossing' is an insertion indicator.
'good man' becomes 'st'.
'hay' placed around 'st' is 'HASTY'.
(Other definitions for hasty that I've seen before include "Precipitate" , "Rashly quick" , "Reckless; hurried" , "Rushed; rash" , "Impetuous - quick-tempered" .)