Cost of keeping soldiers in step (7)
I believe the answer is:
storage
'cost of keeping' is the definition.
(I've seen this before)
'soldiers in step' is the wordplay.
'soldiers' becomes 'OR' (Other Ranks).
'in' means one lot of letters goes inside another.
'step' becomes 'stage'.
'or' inserted inside 'stage' is 'STORAGE'.
(Other definitions for storage that I've seen before include "Charge for keeping" , "What repository provides; so great (anag.)" , "Cost of keeping in a warehouse" , "Depository space for goods" , "Available space for keeping things" .)