One's promise to uncle? (6)

I believe the answer is:

pledge

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'one's promise' is the definition.
'pledge' can be an answer for 'promise' (pledge is a kind of promise). I'm not certain of the 'one's' bit.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for pledge that I've seen before include "Give one's word" , "make toast" , "Plight" , "Promise - security" , "assurance" .)

I've seen this clue in the Evening Standard.

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