idiom vs like clockwork

idiom is a noun but like clockwork is not a noun.

Word	Noun	Adjective	Verb	Adverb
idiom	Yes	No	No	No
like clockwork	No	No	No	Yes

idiom (noun)	like clockwork (noun)
the style of a particular artist or school or movement
the usage or vocabulary that is characteristic of a specific group of people
an expression whose meanings cannot be inferred from the meanings of the words that make it up
a manner of speaking that is natural to native speakers of a language

idiom (adverb)	like clockwork (adverb)
	with regularity and precision