To lay a charge, soldiers receive a sign (9)
I believe the answer is:
indicator
'sign' is the definition.
(I know that indicator is a type of sign)
'to lay a charge soldiers receive a' is the wordplay.
'to lay a charge' becomes 'indict' (I can't explain this - if you can you should believe this answer much more).
'soldiers' becomes 'OR' (Other Ranks).
'receive' is an insertion indicator.
'indict'+'or'='indictor'
'indictor' enclosing 'a' is 'INDICATOR'.
Can you help me to learn more?
(Other definitions for indicator that I've seen before include "sign" , "one might be on the blink?" , "shower" , "Station information board" , "Car's flashing light" .)