21 across
Mass
of
people
in
lorry,
the
first
to
leave
(4)
Mass of people in lorry, the first to leave (4)
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The definition is at the beginning of the clue.
How the clue works
The definition is Mass of people — a ruck being a crowd or throng. Think of the word for a lorry, TRUCK, then do what the clue says and let "the first to leave" — drop that leading T, and you're left with RUCK.