Not all there spreading nepotism among subordinate officers (3,6,6)
I believe the answer is:
non compos mentis
'not all there' is the definition.
(I've seen this in another clue)
'spreading nepotism among subordinate officers' is the wordplay.
I cannot really understand how this works, but
an anagram of 'nepotism' is 'posmenti' which is present in the answer.
'officers' could be 'noncoms' (short for non-commissioned officers) and 'noncoms' is located in the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for non compos mentis that I've seen before include "not quite all there" , "Not in one's right mind" , "crackers" , "Not fit to try" , "mad" .)