This is the Watchwhich I lost. Identify the underlined clause here:

The right answer is: Adjective clause. 

Adjective Clause
An adjective clause, or relative clause, is a type of dependent clause that works to describe a noun in a sentence. It functions as an adjective even though it is made up of a group of words instead of just one word. In the case of an adjective clause, all the words work together to modify the noun or pronoun. 
Example:
- The flowers that we picked up last week have died.