### Clue: Army arrived first in this, secretly (6)
### Answer: CAMERA 
 ### Explanation: Definition: in this, secretly
  CAME(arrived) **placed before**(first) RA(abbrev. of $$R$$oyal $$A$$rtillery, branch of the British army)
Defn:  "In camera", Neo-Latin for "in the chamber", ie. in the privacy,secretly perhaps, of a judge's chamber, as opposed to "in open court".