Body of troops (8)
'body of troops' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for corps that I've seen before include "Part of an army" , "Large body of troops" , "Specialised mil. body" , "ballet group" , "Body of men" .)
(Other definitions for regiment that I've seen before include "Organise; army unit" , "Large army unit" , "Group of battalions" , "Brigade" , "subject to excessive control" .)