Slip up, with some improper response (3)
I believe the answer is:
err
'slip up' is the definition.
(to err is to make an error)
'with some improper response' is the wordplay.
'with' indicates the answer is hidden within the clue.
'ERR' is hidden in the letters of 'some improper response'.
(Other definitions for err that I've seen before include "Drop a clanger" , "Slip up" , "'To . . . is human, to forgive divine'" , "Sin" , "'to... is human,to forgive divine'" .)