Troop assembled by army officer as per procedure (8)
I believe the answer is:
protocol
'procedure' is the definition.
(I've seen this before)
'troop assembled by army officer as per' is the wordplay.
I cannot really understand how this works, but
an anagram of 'troop' is 'proto' which is within the answer.
'officer' could be 'col' (short for colonel) and 'col' is found in the answer.
This accounts for all the letters.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for protocol that I've seen before include "Code of behaviour" , "Code of correct conduct" , "Poor clot involved in kind of status etiquette" , "Diplomatic rules" , "Formal code" .)