Fed up with editor after snack's licked (8)
I believe the answer is:
defeated
'licked' is the definition.
(thesaurus)
'fed up with editor after snack's' is the wordplay.
'fed up' becomes 'def' ('fed' going up in a down clue).
'with' means one lot of letters go next to another.
'editor' becomes 'ed'.
'after' means one lot of letters go next to another.
'snack' becomes 'eat' (eat can mean a snack).
'ed' put after 'eat' is 'eated'.
'def'+'eated' is 'DEFEATED'.
(Other definitions for defeated that I've seen before include "Did cream" , "Won victory over" , "thwarted" , "Vanquished" , "Beat; beaten" .)