In hospital, immersed in a large volume (2,4)
I believe the answer is:
at home
'in' is the definition.
(I've seen this before)
'hospital immersed in a large volume' is the wordplay.
'hospital' becomes 'h'.
'immersed in' means one lot of letters goes inside another.
'large volume' becomes 'tome' (I've seen this before).
'a'+'tome'='atome'
'h' put within 'atome' is 'AT-HOME'.
(Other definitions for at home that I've seen before include "celebration" , "Reception in one's own abode" , "In one's own house" , "Comfortable - in one's own country" , "Oh mate! (anag)" .)