Crossword Genius

Light lifting device round channel navy used to hold tons (4-1-7)

Ross

I believe the answer is:

jack-o-lantern

But I can't explain it at all!

(Another definition for jack-o-lantern that I've seen is " Will-o'-the-wisp (4-1'-7)".)

I've seen this clue in The Times.
Want a hint initially instead of a full solution? Install my app