Major road blocked by hostile heavy guns (9)
I believe the answer is:
artillery
'heavy guns' is the definition.
(I've seen this before)
'major road blocked by hostile' is the wordplay.
'major road' becomes 'artery' (I've seen this before).
'blocked by' is an insertion indicator (I've seen this in other clues).
'hostile' becomes 'ill' ('ill' can be similar in meaning to 'hostile').
'artery' going around 'ill' is 'ARTILLERY'.
(Other definitions for artillery that I've seen before include "Regiment" , "Army unit that uses big guns" , "Guns, cannons etc" , "Heavy weapons" , "Gunners" .)