Investigator's prime article that may upset diplomatic aide (8,7)
I believe the answer is:
military attache
'diplomatic aide' is the definition.
(military attaches do diplomatic work in an army)
'investigator's prime article that may upset' is the wordplay.
'prime' indicates taking the first letters (prime can mean first).
'upset' indicates an anagram.
The initial letter of 'investigators' is 'i'.
'i'+'article'+'that'+'may'='iarticlethatmay'
'iarticlethatmay' anagrammed gives 'MILITARY ATTACHE'.
(Another definition for military attache that I've seen is " army officer serving with an embassy".)