Return part of allowance that's missing (4)
I believe the answer is:
awol
'missing' is the definition.
(military abbreviation for absent without leave)
'return part of allowance that's' is the wordplay.
'return' is a reversal indicator.
'part of' indicates the answer is hidden within the clue.
'lowa' can be found hidden inside 'allowance that'.
'lowa' written backwards gives 'AWOL'.
(Other definitions for awol that I've seen before include "Army offence of being missing from duty, briefly" , "Absent without permission, in short" , "'Absent without permission, initially (4)'" , "gone missing" , "On the lam" .)