Undertaking to provide security at uncle's (6)

I believe the answer is:

pledge

I'm a little stuck... Click here to teach me more about this clue!

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

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

(Other definitions for pledge that I've seen before include "Plight" , "Covenant" , "Solemn promise" , "Firm commitment" , "Promise, oath" .)

I've seen this clue in the Evening Standard.

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