Academy founder, name inspiring old military unit (7)
I believe the answer is:
platoon
'military unit' is the definition.
(I know that platoon is a type of army unit)
'academy founder name inspiring old' is the wordplay.
'academy founder' becomes 'plato' (I've seen this in another clue).
'name' becomes 'n' (common abbreviation e.g. on forms).
'inspiring' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'plato'+'n'='platon'
'platon' going around 'o' is 'PLATOON'.
(Other definitions for platoon that I've seen before include "Group of soldiers" , "detail" , "Division of company" , "Small military unit" , "Company's subdivision" .)