Honour age, returning last letter (5)
I believe the answer is:
omega
'last letter' is the definition.
(last letter of the Greek alphabet)
'honour age returning' is the wordplay.
'honour' becomes 'om' (Order of Merit).
'returning' is a reversal indicator.
'age' written backwards gives 'ega'.
'om'+'ega'='OMEGA'
(Other definitions for omega that I've seen before include "Conclusion" , "Athenian character" , "The last in a series of letters" , "Foreign character" , "Ultimate letter" .)