Absent without leave (4)
I believe the answer is:
awol
'absent without leave' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for awol that I've seen before include "gone missing" , "Truant (or just missing)" , "Take unofficial leave (abbrev.)" , "Absent without leave (abbrev.)" , "Absent without leave (abbr.)" .)