Bright person drops kilo in box (4)

I believe the answer is:

spar

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

'box' is the definition.
(sparring is a kind of boxing)

I cannot understand how the remainder of the clue works.

(Other definitions for spar that I've seen before include "Close friend" , "Box in training - nautical pole" , "Stout pole, nautically" , "Do up (bird for oven)" , "Pole-shaped part of the rigging" .)

I've seen this clue in The Sun.

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.