Like model in Hartley; ends impulsive (5)
I believe the answer is:
hasty
'impulsive' is the definition.
(I've seen this before)
'like model in hartley ends' is the wordplay.
'like' becomes 'as'.
'model' becomes 't' (Ford Model T car).
'in' is an insertion indicator.
'ends' says to hollow out the word (remove centre letters).
'hartley' with its middle taken out is 'hy'.
'as'+'t'='ast'
'ast' going inside 'hy' is 'HASTY'.
(Other definitions for hasty that I've seen before include "Rash; thoughtless" , "Precipitate" , "Impulsive" , "Impetuous - quick-tempered" , "Too quick, impetuous" .)