Command one army corps, in a manner of speaking (9)
I believe the answer is:
direction
'command' is the definition.
(direction is a kind of command)
'one army corps in a manner of speaking' is the wordplay.
'one army corps' becomes 're' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'a manner of speaking' becomes 'diction' (I've seen this before).
're' inserted inside 'diction' is 'DIRECTION'.
(Other definitions for direction that I've seen before include "The way in which something is moving" , "Command" , "Guidance" , "Course in which something is travelling" , "Administration" .)