Ordnance is harmless in main transport channel (6)

I believe the answer is:

artery

'main transport channel' is the definition.
(arteries transport blood)

I can't explain the remainder of the clue.

(Other definitions for artery that I've seen before include "main line" , "blood line" , "Vessel carrying blood from heart" , "Major traffic route" , "Blood-vessel" .)

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app