Important transport link cut by hostile heavy guns (9)
I believe the answer is:
artillery
'heavy guns' is the definition.
(I've seen this before)
'important transport link cut by hostile' is the wordplay.
'important transport link' becomes 'artery' (artery can mean a vital transport route).
'cut by' is an insertion indicator.
'hostile' becomes 'ill' (similar in meaning).
'artery' enclosing 'ill' is 'ARTILLERY'.
(Other definitions for artillery that I've seen before include "group of soldiers" , "Army unit that uses big guns" , "Regiment" , "Heavy weapons" , "Heavy guns" .)