What is the grammatical analysis of 信得过? 信 = to believe.
得过 = ???

Is this construction a sentence, a phrase, a verbalphrase or maybe sth. different?
信得过 means 'can be trusted/believed/depended upon'. I would actually classify it as a verbal phrase as there are usually more formal ways to express it in writing, such as 可靠 (dependable) or 可信 (believable). 

I would say “他的话是信得过的！” if I wanted to say "His word could be trusted!"