Left the armed forces without permission (8)
I believe the answer is:
deserted
'left the armed forces without permission' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for deserted that I've seen before include "Left the armed forces illegally" , "fled" , "Defected, renounced" , "defected,renounced" , "Left the armed forces without permission" .)