Mistake by army officer brings ruin (8)
I believe the answer is:
collapse
'ruin' is the definition.
(I've seen this before)
'mistake by army officer' is the wordplay.
'mistake' becomes 'lapse' (lapse is a kind of mistake**).
'by' is a charade indicator (letters next to each other).
'army officer' becomes 'col' (I've seen this in other clues).
'lapse' put after 'col' is 'COLLAPSE'.
'brings' acts as a link.
(Other definitions for collapse that I've seen before include "Fall down unconscious" , "Give way or cave in" , "Founder" , "Slump" , "Fall down or cave in" .)