Furniture item found in thousands of apartments (4)
I believe the answer is:
sofa
'furniture item' is the definition.
(sofa is a type of furniture)
'in thousands of apartments' is the wordplay.
'in' indicates a hidden word.
'SOFA' can be found hidden inside 'thousands of apartments'.
'found' is the link.
(Other definitions for sofa that I've seen before include "Upholstered seat for more than one person" , "Settee" , "Long upholstered seat with back and arms" , "Area for sitting" , "Wide padded seat" .)