sing vs let on

sing and let on both are verbs.

Word	Noun	Adjective	Verb	Adverb
sing	No	No	Yes	No
let on	No	No	Yes	No

As verbs, let on is a hypernym of sing; that is, let on is a word with a broader meaning than sing:

sing: divulge confidential information or secrets
let on: make known to the public information that was previously known only to a few people or that was meant to be kept a secret

Other hypernyms of sing include break, bring out, disclose, discover, divulge, expose, give away, let out, reveal, uncover, unwrap.

sing (verb)	let on (verb)
make a whining, ringing, or whistling sound	make known to the public information that was previously known only to a few people or that was meant to be kept a secret
divulge confidential information or secrets
to make melodious sounds
produce tones with the voice
deliver by singing