Bulb – its brightness? (4)

I believe the answer is:

lamp

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

'bulb' is the definition.
(I know that bulb is a type of bulb)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for lamp that I've seen before include "Source of light, usually portable" , "Florence Nightingale was the Lady with the . . . ." , "Device for giving light on palm" , "Appliance producing light" , "punch" .)

I've seen this clue in The Guardian.

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.