Disappointed -- at being dropped?

I believe the answer is:

let down

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

I don't understand the remainder of the clue.

(Other definitions for let down that I've seen before include "Disappointment < lowered" , "Disappoint, betray" , "Fail to meet expectations of others" , "blow" , "Failed - deflated" .)

I've seen this clue in The Telegraph.

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