Creep's bitterness (4)
I believe the answer is:
edge
This clue is a double definition.
'creep's' is the first definition.
(I've seen this in other clues)
'bitterness' is the second definition.
(thesaurus)
(Other definitions for edge that I've seen before include "Final line" , "Rim, lip" , "Advantage (informal)" , "Verge" , "Keenness" .)