A boom in shipping perhaps (4)

I believe the answer is:

spar

I'm a little stuck... Click here to teach me more about this clue!

'a boom' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

(Other definitions for spar that I've seen before include "Crystal - box" , "Pole to support sails, rigging etc" , "Shadow-box" , "Yard" , "Box without punching hard" .)

I've seen this clue in The Globe and Mail.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.