Drug addict follows mother with a gun (6)
I believe the answer is:
mauser
'a gun' is the definition.
(I've seen this before)
'drug addict follows mother' is the wordplay.
'drug addict' becomes 'user' (I've seen this before).
'follows' says to put letters next to each other.
'mother' becomes 'ma' (ma is a kind of mother).
'user' put after 'ma' is 'MAUSER'.
'with' acts as a link.
(Other definitions for mauser that I've seen before include "gun" , "rifle" .)