It's worth noting for newcomers that Boatman is a Guardian crossword compiler (see 25590 for example) hence the capital B. 19A must surely be PUN. Clue seems rather tortuous, but less so than 13A.
(Desert = definition, rat returning = tar = AB (Able Seaman), A, North Dakota = ND, 9 after vacation = OPERATION with inner letters removed = ON.)