The head? Impetuous? Rubbish (5)
I believe the answer is:
trash
'rubbish' is the definition.
(I know that rubbish can be written as trash)
'the head? impetuous?' is the wordplay.
'head?' says to take the initial letters (I've seen 'head' mean this).
'impetuous?' becomes 'rash' (I've seen this before).
The first letter of 'the' is 't'.
't'+'rash'='TRASH'
(Other definitions for trash that I've seen before include "pulp" , "Vandalise" , "Rubbish - destroy" , "Junk" , "Wreck; rubbish" .)