Nervous, and liable to fall? (2,4)

I believe the answer is:

on edge

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'nervous' is the definition.
(on edge can mean nervous or tense)

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

Can you help me to learn more?

(Other definitions for on edge that I've seen before include "Highly strung" , "Fidgety, nervous" , "stressed" , "looking down at Much Wenlock?" , "Twitchy" .)

I've seen this clue in The Times.

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