Philosopher working to form military unit (7)
I believe the answer is:
platoon
'military unit' is the definition.
(I know that platoon is a type of army unit)
'philosopher working' is the wordplay.
'philosopher' becomes 'Plato' (Plato is an example).
'working' becomes 'on' (e.g. a machine is working or on).
'plato'+'on'='PLATOON'
'to form' is the link.
(Other definitions for platoon that I've seen before include "detail" , "Infantry unit" , "Military group" , "part of company" , "Group of soldiers" .)