Signal for pick-me-up? (5)

I believe the answer is:

thumb

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'signal for pick-me-up?' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.

This is the entire clue.

Can you help me to learn more?

(Other definitions for thumb that I've seen before include "Hitch ride" , "Indicate need for lift" , "Hitch-hiker's tool" , "Tom ___ , traditional hero of English folklore" , "Second part of 12" .)

I've seen this clue in The Telegraph.

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