Big guns badly stuck in main road (9)
I believe the answer is:
artillery
'big guns' is the definition.
(I've seen this before)
'badly stuck in main road' is the wordplay.
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'stuck in' means one lot of letters goes inside another.
'main road' becomes 'artery'.
'ill' put into 'artery' is 'ARTILLERY'.
(Other definitions for artillery that I've seen before include "Heavy weapons" , "Army unit that uses big guns" , "Ordnance" , "group of soldiers" , "Large guns for use on land" .)