What does the cryptic crossword clue "Half of that rift zone is rocky non-functional (10)" mean?

The wordplay uses an anagram. anagram of TH (synonym="TH") + RIFT (synonym="RIFT") + ZONE (synonym="ZONE") = ONTHEFRITZ The answer is ONTHEFRITZ.

Half of that rift zone is rocky non-functional (10)

Independent Cryptic #11672 | 14 Down | 2024-03-08 flower