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 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" .)