The last word in mattresses on which we'll be completely out

'the last word in mattresses on which' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.

'we'll be completely out' is the wordplay.
'we'll' becomes 'beam' (I can't explain this - if you can you should believe this answer much more).
'be' becomes 'end' (ending is a kind of being).
'completely' becomes 'so' (both can mean 'very much').
'out' means to remove the last letter.
'so' with its last letter taken off is 's'.
'beam'+'end'+'s'='BEAM-ENDS'

Can you help me to learn more?