That is to accommodate elderly relative -- it's vacant (5)
I believe the answer is:
inane
'vacant' is the definition.
(thesaurus)
'that is to accommodate elderly relative' is the wordplay.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'to accommodate' indicates putting letters inside.
'elderly relative' becomes 'nan' (I've seen this before).
'ie' going around 'nan' is 'INANE'.
'it's' is the link.
(Other definitions for inane that I've seen before include "Foolish, fatuous" , "Thick" , "Delirious" , "Lacking sense, silly" , "Without meaning or point" .)