Old Testament prophet who denounced Ahab and Jezebel (6)
I believe the answer is:
elijah
(Other definitions for elijah that I've seen before include "Hebrew prophet who rebuked Ahab and Jezebel" , "Mendelssohn oratorio" , "Hebrew prophet in the time of Jezebel" , "Old Testament prophet" , "O.T. prophet who disappeared in fiery chariot" .)