Capitals of California and Nevada accommodating State Chamber (6)
I believe the answer is:
cavern
'chamber' is the definition.
(I've seen this before)
'capitals of california and nevada accommodating state' is the wordplay.
'capitals of' says to take the initial letters.
'and' means one lot of letters go next to another.
'nevada' becomes 'n' (this could be a standard abbreviation I've not previously seen).
'accommodating' is an insertion indicator.
'state' becomes 'aver' (both can mean to declare).
The initial letter of 'california' is 'c'.
'c'+'n'='cn'
'cn' placed around 'aver' is 'CAVERN'.
Can you help me to learn more?
(Other definitions for cavern that I've seen before include "Large 24 chamber" , "Liverpool club where The Beatles started out" , "Large underground hollow" , "Vast dark space" , "Detection device" .)