Carry out order causing catastrophe (4)
I believe the answer is:
doom
'catastrophe' is the definition.
(I've seen this before)
'carry out order' is the wordplay.
'carry out' becomes 'do' ('do' can be a synonym of 'carry out').
'order' becomes 'om' (order of merit).
'do'+'om'='DOOM'
'causing' is the link.
(Other definitions for doom that I've seen before include "Mood of death or ruin" , "Final ruin" , "Disastrous fate" , "Inevitable ruin" , "Inescapable fate" .)