Traverse part of London for weapon (8)
I believe the answer is:
crossbow
'for weapon' is the definition.
(crossbow is a kind of weapon)
'traverse part of london' is the wordplay.
'traverse' becomes 'cross' (synonyms).
'part of london' becomes 'Bow' (area of East London).
'cross'+'bow'='CROSSBOW'
(Other definitions for crossbow that I've seen before include "Missile-launcher" , "One responsible for shooting bolts" , "Old weapon of war" , "Weapon for shooting bolts" , "Weapon firing bolts" .)