system clock vs clock

system clock and clock both are nouns.

Word	Noun	Adjective	Verb	Adverb
system clock	Yes	No	No	No
clock	Yes	No	Yes	No

As nouns, clock is a hypernym of system clock; that is, clock is a word with a broader meaning than system clock:

system clock (noun)	clock (noun)
an electronic device in a computer that issues a steady high-frequency signal that synchronizes all the internal components	a timepiece that shows the time of day
a time-of-day clock in a computer system

system clock (verb)	clock (verb)
	measure the time or duration of an event or action or the person who performs an action in a certain period of time