Cabbage-moth's eating up last letter (5)
I believe the answer is:
omega
'last letter' is the definition.
(last letter of the Greek alphabet)
'cabbage-moth's eating up' is the wordplay.
'eating' says the answer is hidden in the clue.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'agemo' can be found hidden inside 'cabbage moths'.
'agemo' reversed gives 'OMEGA'.
(Other definitions for omega that I've seen before include "End of sequence" , "character from Athens" , "Final Greek letter" , "Athenian character" , "letter to Greeks" .)