Making a mistake in the German gun (9)
I believe the answer is:
derringer
'gun' is the definition.
(derringer is a kind of gun)
'making a mistake in the german' is the wordplay.
'making a mistake' becomes 'erring' ('err' can be a synonym of 'make a mistake').
'in' indicates putting letters inside.
'the german' becomes 'der' ('the' in German).
'erring' placed into 'der' is 'DERRINGER'.
(Other definitions for derringer that I've seen before include "Short-barrelled American pistol" , "Pocket pistol" , "Weapon" , "Small pistol used by gamblers in western films" .)