Heavy blow; political influence (5)
I believe the answer is:
clout
I believe this is a double definition.
'heavy blow' is the first definition.
(I've seen this before)
'political influence' is the second definition.
(I've seen this in another clue)
(Other definitions for clout that I've seen before include "Hit - influence" , "Smack - influence" , "Punch; influence" , "authority" , "piece of clothing" .)