Relaxed with a big book to bury his head in (2,4)

I believe the answer is:

at home

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'in' is the definition.
(someone at home is 'in')

'relaxed with a big book to bury his head' is the wordplay.
'relaxed with' becomes 'a' (I am not sure about this - if you are sure you should believe this answer much more).
'a big book' becomes 'tome' (term for a large or heavy book).
'to bury' is an insertion indicator (inserted letters are buried inside).
'head' indicates taking the first letters.
The first letter of 'his' is 'h'.
'tome' enclosing 'h' is 'thome'.
'a'+'thome'='AT-HOME'

Can you help me to learn more?

(Other definitions for at home that I've seen before include "celebration" , "In (for a party?)" , "Reception in one's own abode" , "ready to receive visitors" , "Comfortable - in one's own country" .)

I've seen this clue in the Sydney Morning Herald.

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