cling vs hold on

cling is a noun but hold on is not a noun.

Word	Noun	Adjective	Verb	Adverb
cling	Yes	No	Yes	No
hold on	No	No	Yes	No

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

Other hypernyms of cling include grasp.

cling (noun)	hold on (noun)
fruit (especially peach) whose flesh adheres strongly to the pit

cling (verb)	hold on (verb)
come or be in close contact with; stick or hold together and resist separation	be persistent, refuse to stop
hold on tightly or tenaciously	stop and wait, as if awaiting further instructions or developments
to remain emotionally or intellectually attached	hold the phone line open
	hold firmly
	retain possession of