A gee-gee pinned by the German weapon (6)
I believe the answer is:
dagger
'weapon' is the definition.
(dagger is a kind of weapon)
'a gee-gee pinned by the german' is the wordplay.
'gee' becomes 'g' (synonyms).
'gee' becomes 'g' (synonyms).
'pinned by' indicates putting letters inside.
'the german' becomes 'der' ('the' in German).
'a'+'g'+'g'='agg'
'agg' placed into 'der' is 'DAGGER'.
(Other definitions for dagger that I've seen before include "Hand weapon" , "Macbeth's weapon" , "Murder weapon" , "Short knife for stabbing" , "The weapon Macbeth thought he saw" .)