Last home custom-built? Not so much (4)
I believe the answer is:
tomb
'last home' is the definition.
(thesaurus)
'custom-built? not so much' is the wordplay.
'not so much' indicates a hidden word (only some of the letters).
'TOMB' can be found hidden inside 'custom built'.
(Other definitions for tomb that I've seen before include "Sepulchre" , "grave situation" , "Underground vault" , "Large vault" , "Last resting-place" .)