Lower due to a setback? (3,4)

I believe the answer is:

let down

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'lower' is the definition.
(synonyms)

I don't understand the remainder of the clue.

(Other definitions for let down that I've seen before include "Failed - to deflate a tyre - disappointed" , "Disappoint or fail - or lengthen garment" , "Disappoint, betray" , "lengthened" , "Disappoint, fail to support" .)

I've seen this clue in The Globe and Mail.

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