It must be back before the tenth (5)
I believe the answer is:
tithe
'tenth' is the definition.
(a tenth of one's income given to a church)
'it must be back before the' is the wordplay.
'must be back' is a reversal indicator (I've seen 'must be backed' mean this).
'before' says to put letters next to each other.
'it' written backwards gives 'ti'.
'ti'+'the'='TITHE'
(Other definitions for tithe that I've seen before include "Levy by church, e.g." , "Ancient tax" , "Tax to support the church" , "financial contribution" , "One-tenth; church tax" .)