What does the cryptic crossword clue "Rapid movement inside hill, a result of fracking? (6)" mean?

The wordplay uses a container (one word inside another). REM (synonym="Rapid Eye Movement") in TOR (synonym="hill") = TREMOR The answer is TREMOR.

Rapid movement inside hill, a result of fracking? (6)

Guardian Cryptic #29048 | 14 Across | 2023-04-19 flower