Soldiers may be told to stand thus for a piece of cake (4)
I believe the answer is:
easy
'piece of cake' is the definition.
(I've seen this in another clue)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for easy that I've seen before include "Gentle - light" , "without difficulty" , "Frederick Marryat's midshipman" , "Undemanding" , "Requiring little effort" .)