Protective cover (6)
(Other definitions for casing that I've seen before include "Recceing" , "Protective covering as of engine" , "Outside cover" , "Outer covering of something" , "This gives protection" .)
(Other definitions for armour that I've seen before include "Chain mail, e.g." , "Soldier's body protection" , "Metal or chainmail suit" , "Protective covering made of metal" , "Greave, sallet and cuirass are pieces of this" .)
'protective cover?' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alias that I've seen before include "Pseudonym" , "Also known as" , "Alternative name" , "False identity" , "aka" .)