What does the cryptic crossword clue "Shake like 16 down over first meeting (7)" mean?

The definition part of the clue is "Shake". The answer is TREMBLE. Explanation: M[eeting] in TREBLE

Shake like 16 down over first meeting (7)

Guardian Puzzle #26569 20 Down 2015-05-12

Definition

Shake

Explanation

M[eeting] in TREBLE

Source: Fifteensquared

Answer

TREMBLE