Traverse part of London with a weapon (8)
I believe the answer is:
crossbow
'a weapon' is the definition.
(I know that crossbow is a type of bow)
'traverse part of london' is the wordplay.
'traverse' becomes 'cross' (synonyms).
'part of london' becomes 'Bow' (area of East London).
'cross'+'bow'='CROSSBOW'
'with' acts as a link.
(Other definitions for crossbow that I've seen before include "Old weapon with mechanism for firing arrow" , "Ancient weapon, shot bolts" , "Missile-launcher" , "can precipitate a quarrel" , "historic weapon" .)