Beneath heading in Chambers he’s about to look up ‘mousetrap’ say (6)

I believe the answer is:
cheese
'mousetrap say' is the definition.
The definition and answer can be both to do with fundamental existence as well as being verbs in their base form.
Perhaps you can see a link between them that I don't see?
'beneath heading in chambers he's about to look up' is the wordplay.
'beneath' says to put letters next to each other (in a down clue).
'heading in' says to take the initial letters (I've seen 'headings in' mean this).
'about' indicates putting letters inside.
'to look' becomes 'see' ().
'up' says the letters should be written backwards (in down clue: letters go upwards).
The first letter of 'chambers' is 'c'.
'see' in reverse letter order is 'ees'.
'he' going around 'ees' is 'heese'.
'c' next to 'heese' is 'CHEESE'.
Can you help me to learn more?
(Other definitions for cheese that I've seen before include "Nautical peaked cap - board with wire attached" , "A big one would be important" , "Soft or more solid creamy food" , "Wensleydale, Stilton etc" , "Nautical cap worn without badge" .)
