let down vs reef

let down is not a noun while reef is a noun.

Word	Noun	Adjective	Verb	Adverb
let down	No	No	Yes	No
reef	Yes	No	Yes	No

As verbs, reef is a hyponym of let down; that is, reef is a word with a more specific, narrower meaning than let down:

Other hyponyms of let down include depress, dip, incline.

let down (noun)	reef (noun)
	one of several strips across a sail that can be taken in or rolled up to lessen the area of the sail that is exposed to the wind
	a submerged ridge of rock or coral near the surface of the water

let down (verb)	reef (verb)
fail to meet the hopes or expectations of	roll up (a portion of a sail) in order to reduce its area
move something or somebody to a lower position	reduce (a sail) by taking in a reef
	lower and bring partially inboard