12 across
Tiny
amount,
nothing
to
plug
hole
in
the
wall
(4)
Tiny amount, nothing to plug hole in the wall (4)
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The definition is at the beginning of the clue.
How the clue works
The definition is Tiny amount, meaning a very small quantity. For the wordplay, take ATM (hole in the wall, your cash machine) and plug O (nothing) inside it to get A-T-O-M.