Bodily cavity, pulmonary organ almost filled by yours truly (5)
I believe the answer is:
lumen
'bodily cavity' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'pulmonary organ almost filled by yours truly' is the wordplay.
'pulmonary organ' becomes 'lung' (I've seen this in another clue).
'almost' means to remove the last letter (almost the whole word is kept).
'filled by' means one lot of letters goes inside another.
'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" .)