Philosopher writing about military group (7)
I believe the answer is:
platoon
'military group' is the definition.
(I know that platoon is a type of army unit)
'philosopher writing about' is the wordplay.
I cannot really see how this works, but
'philosopher' could be 'plato' (Plato is an example) and 'plato' is found within the answer.
'about' could be 'on' (as in a book 'on' a certain topic) and 'on' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for platoon that I've seen before include "Military group" , "Sub-unit of a military company" , "Team under lieutenant" , "Infantry unit" , "Small military unit" .)