Disappointment with puncture? (3-4)

I believe the answer is:

let down

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'disappointment' 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 "Aircraft descent" , "Disappoint, betray" , "disillusionment" , "Disappoint, fail to keep promise" , "Failed - deflated" .)

I've seen this clue in The Telegraph.

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