part of speech of the idiom "come clean"

From the novel adapted from the British political comedy Yes, Minister:

Sir Humphrey then came clean, rather reluctantly.

Is it possible to analyze the part of speech of both come and clear here?

1. come is similar to a copula and clean is an adjective, the complement
2. come is a verb and clean is an adverb of manner

Reference:

WordReference thinks come in the context above is an intransitive verb.

Analysing the internal structure of this idiom isn't very helpful. It's an idiom and can't be used to construct new expressions.

"Clean" is normally an adjective. That analysis works here so there isn't any need to look further. "Come" is therefore a linking verb. There are a few other idioms like this with "come". I can think of "come good" and "come true"

Linking verbs are intransitive in the sense that the complement (noun, adjective, etc) can't be made the subject in a passive sentence. If you have "The painting looks beautiful", you can't form a passive *Beautiful is looked by the painting". The complement "beautiful" isn't an object, and so the verb isn't transitive.