Genial expression of self-doubt? (7)

I believe the answer is:

amiable

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'genial' is the definition.
(I know that genial can be written as amiable)

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

Can you help me to learn more?

(Other definitions for amiable that I've seen before include "Charming" , "'Affable, genial (7)'" , "Lovable, friendly" , "Friendly and good-natured (7)" , "'Friendly, pleasant and good-tempered (7)'" .)

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