This is a container clue - one word or phrase is placed inside another word to form the answer.
Look for a word meaning 'barrel' that contains a word meaning 'dark side' (think of the philosophical concept of opposing forces). The container word wraps around the contained word.
Answer: KEYING | YIN (dark side, as in yin and yang) goes inside KEG (barrel) to give K-YIN-EG → KEYING (scratching surface)