Grouped in threes with Military Intelligence complete (9)
I believe the answer is:
terminate
'complete' is the definition.
(thesaurus)
'grouped in threes with military intelligence' is the wordplay.
'grouped in threes' becomes 'ternate' (I am not sure about this - if you are sure you should believe this answer much more).
'with' is an insertion indicator.
'military intelligence' becomes 'MI' (as in MIa or MIt - British military intelligence).
'ternate' going around 'mi' is 'TERMINATE'.
Can you help me to learn more?
(Other definitions for terminate that I've seen before include "Stop" , "Cut short" , "Put an end to, finish" , "Conclude" , "Put an end to, discontinue" .)