Republic's author about to abandon army group (5)
I believe the answer is:
plato
'republic's author' is the definition.
(author of Republic)
'about to abandon army group' is the wordplay.
'about' becomes 'on'.
'to abandon' indicates named letters should be taken away.
'army group' becomes 'platoon'.
'platoon' with 'on' taken away is 'PLATO'.
(Other definitions for plato that I've seen before include "Republican writer?" , "Greek philosopher (The Republic)" , "Moon crater named after Greek philosopher" , "Teacher of Aristotle, pupil of Socrates" , "Ancient Athenian philosopher, pupil of Socrates" .)