As he is ill, he cannot come. The underlined clause is a/an-

An adverbial clause is a group of words that functions as an adverb in a sentence.
The clause can modify or describe verbs, adverbs and adjectives.
In the given sentence, the clause ‘As he is ill’ describes the reason for the action of the verb ‘(not) come’. So, it is an adverbial clause.
Source: dictionary.com