THE 5-LETTER WORD FOR "TO LET DOWN" IS OFTEN:

DISAP (Short for DISAPPOINT) However, DISAP is not a standard full word on its own. A better, valid 5-letter synonym for "to let down" is: FAILS Explanation: Fails means to let someone down by not meeting expectations or not succeeding. Example Sentence: He fails to deliver on his promise and lets everyone down.