Rennet is an extract made from the lining of a calf's fourth stomch.

Question

In the sentence above, what is the verb and is it linking or action?

I thought it would be an action verb-is made.But I am not sure.

The verb in the main clause is IS.

Rennnet IS an extract. It's a linking verb -- because rennet and extract are the same thing.

MADE is a participle. The participial phrase made from the lining of a calf's fourth stomach describes extract.