"a/the Fly in the __"

This page is the CodyCross answer for the clue below from Inventions, Group 51, Puzzle 2.

Question is: "a/the Fly in the __"

and the answer is: OINTMENT

Word Meaning:

OINTMENT — (medicine) A viscous preparation of oils and/or fats, usually containing medication, used as a treatment or as an emollient.

Definitions from Wiktionary.