Seem ready to drop off (3)

I believe the answer is:

nod

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'drop off' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

(Other definitions for nod that I've seen before include "Signal with the head" , "Signify agreement" , "Indicate assent; make slip" , "Shake head in agreement" , "Move head in agreement" .)

I've seen this clue in the Evening Standard.

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