Unit of separation (10)
I believe the answer is:
detachment
I believe this is a double definition.
'unit' is the first definition.
(detachment is a kind of unit)
'separation' is the second definition.
(detachment is a kind of separation)
'of' acts as a link.
(Other definitions for detachment that I've seen before include "inclination to keep to the objective" , "Disengagement" , "some engaged in military action?" , "Coolness - body of troops" , "squad" .)