Reserve military unit (10)
I believe the answer is:
detachment
This clue is a double definition.
'reserve' is the first definition.
(I've seen this before)
'military unit' is the second definition.
(detachment is a kind of military unit)
(Other definitions for detachment that I've seen before include "Aloofness" , "squad" , "inclination to keep to the objective" , "some engaged in military action?" , "Disengagement" .)