Nautical Vessel That Radiates a Bright Beam
Puzzle 15 of Running a Marathon Pack
Question is: Nautical Vessel That Radiates a Bright Beam
and the answer is: LIGHTSHIP
Word Meaning:
LIGHTSHIP — (nautical) A ship riding at anchor and displaying a light for the guidance of sailors, in a position where a fixed lighthouse structure would be impracticable.
Definitions from Wiktionary.