Cost of a military action? (6)
I believe the answer is:
charge
This clue is a double definition.
'cost' is the first definition.
(charge is a kind of cost)
'a military action?' is the second definition.
(charge is a kind of military action)
'of' is the link.
(Other definitions for charge that I've seen before include "Attack - require payment" , "Storm" , "Demand in payment" , "offensive, > explosive > tax" , "Move quickly and violently" .)