Rug is housemate’s in part (3)
I believe the answer is:
mat
'rug' is the definition.
(both are floor coverings)
'housemate's in part' is the wordplay.
'in part' says the answer is hidden in the clue.
'MAT' is hidden in the letters of 'housemates'.
'is' acts as a link.
(Other definitions for mat that I've seen before include "Small carpet" , "Unglossy floor pad" , "Tangle" , "Small protective floor-covering" , "Dull, rug" .)