Crossword Genius

Bodily cavity, pulmonary organ almost filled by yours truly (5)

Ross

I believe the answer is:

lumen

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

'bodily cavity' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.

'pulmonary organ almost filled by yours truly' is the wordplay.
'pulmonary organ' becomes 'lung' (I've seen this before).
'almost' means to remove the last letter (almost the whole word is kept).
'filled by' is an insertion indicator.
'yours truly' becomes 'me' (the person who set this crossword).
'lung' with its final letter removed is 'lun'.
'lun' enclosing 'me' is 'LUMEN'.

Can you help me to learn more?

(Other definitions for lumen that I've seen before include "Unit of light flux" , "Unit of luminous flux" .)

I've seen this clue in The Times.
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.