Revealed what was bad about an army corps (5)
I believe the answer is:
bared
'revealed' is the definition.
(I've seen this in another clue)
'bad about an army corps' is the wordplay.
'about' indicates putting letters inside.
'an army corps' becomes 'RE' (Royal Engineers).
'bad' enclosing 're' is 'BARED'.
'what was' is the link.
(Other definitions for bared that I've seen before include "Stripped" , "Uncovered" .)