Boring thing, a pistol taking head off (5)
I believe the answer is:
auger
'boring thing' is the definition.
(tool used for boring)
'a pistol taking head off' is the wordplay.
'pistol' becomes 'luger' (I have seen 'German pistol' mean 'luger' so perhaps 'pistol' could also mean 'luger').
'taking head off' means to remove the first letter.
'luger' with its first letter taken off is 'uger'.
'a'+'uger'='AUGER'
(Other definitions for auger that I've seen before include "A tool for boring hole in wood" , "Tool with spiral point for boring" , "something that's boring" , "Boring-tool" , "Boring instrument" .)