Designate everything that has to be returned (5)
I believe the answer is:
allot
'designate' is the definition.
(I've seen this in another clue)
'everything that has to be returned' is the wordplay.
'everything' becomes 'all' (all things).
'that has' says to put letters next to each other.
'be returned' says the letters should be written backwards (I've seen 'is returned' mean this).
'to' back-to-front is 'ot'.
'all'+'ot'='ALLOT'
(Other definitions for allot that I've seen before include "Hand out" , "Dispense or distribute" , "split up" , "Assign (to someone)" , "Distribute, give as share" .)