Big guns badly interrupting main traffic route (9)
I believe the answer is:
artillery
'big guns' is the definition.
(I've seen this before)
'badly interrupting main traffic route' is the wordplay.
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'interrupting' indicates putting letters inside.
'main traffic route' becomes 'artery' (I've seen this before).
'ill' placed inside 'artery' is 'ARTILLERY'.
(Other definitions for artillery that I've seen before include "Army unit that uses big guns" , "Guns, cannons etc" , "Heavy weapons" , "Large guns for army in land war" , "Missile-launchers and the like" .)