Final answer:
The poem by Victoria Mary Sackville-West uses 'riding-light' to explicitly describe part of a ship. It symbolizes guidance or truth in old age.
Step-by-step explanation:
In the poem by Victoria Mary Sackville-West, the phrase 'Underneath our single riding-light' in the context provided explicitly describes part of a ship.
A riding-light, also known as an anchor light, is a light displayed by ships at anchor.
The poem describes a serene maritime scene and employs this image to create a metaphor for old age being peaceful after the storms of youth.
The riding-light symbolizes the guiding light or truth that may come with old age.