NCO positive soldiers sit around without good reason (3,6,6)
I believe the answer is:
non compos mentis
'without good reason' is the definition.
I can't tell whether this defines the answer.
'nco positive soldiers sit' is the wordplay.
I cannot quite see how this works, but
an anagram of 'nco' is 'onc' which is located in the answer.
'positive' could be 'p' and 'p' is found in the answer.
'soldiers' could be 'men' (men can mean a group of soldiers) and 'men' is found within the answer.
an anagram of 'sit' is 'sti' which is present in the left over letters.
The remaining letters 'nomos' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'around' acts as a link.
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" , "Not fit to try" , "crackers" , "mad" .)