Remains single, investing in building society (5)
I believe the answer is:
bones
'remains' is the definition.
(thesaurus)
'single investing in building society' is the wordplay.
'single' becomes 'one' (similar in meaning).
'investing in' means one lot of letters goes inside another.
'building society' becomes 'BS' (abbreviation for building society).
'one' inserted into 'bs' is 'BONES'.
(Other definitions for bones that I've seen before include "Tibia, fibula etc" , "... skeletons, perhaps" , "Components of skeletons" , "Elements of a skeleton" , "eg Ribs" .)