Letter should give order date for return (5)
I believe the answer is:
omega
'letter' is the definition.
(omega is a kind of letter)
'order date for return' is the wordplay.
'order' becomes 'om' (Order of Merit).
'date' becomes 'age' (both can mean to get older).
'for return' says the letters should be written backwards.
'age' reversed gives 'ega'.
'om'+'ega'='OMEGA'
'should give' is the link.
(Other definitions for omega that I've seen before include "Ending of a series or sequence" , "letter from abroad" , "Last (Gk. letter)" , "Athenian character" , "Final outcome" .)