Line of force (8)
I believe the answer is:
railroad
I believe this clue is a double definition.
'line' is the first definition.
(railroad is a kind of line)
'force' is the second definition.
(railroading is a kind of forcing)
'of' acts as a link.
(Other definitions for railroad that I've seen before include "Unfairly force into decision - way for train" , "Bully" , "[DRAGOON]" , "Push through unduly" , "US term for train line" .)