Truant's old boring tool cases (4)
I believe the answer is:
awol
'truant's' is the definition.
(synonyms)
'old boring tool cases' is the wordplay.
'old' becomes 'o' (abbreviation).
'boring tool' becomes 'awl' (tool for boring holes).
'cases' means one lot of letters goes inside another.
'o' put inside 'awl' is 'AWOL'.
(Other definitions for awol that I've seen before include "Truant (or just missing)" , "Army offence of being missing from duty, briefly" , "Absent without permission, in short" , "gone missing" , "Absent without leave (abbrev.)" .)