Floor regularly covered by endless old-fashioned filth (7)
I believe the answer is:
squalor
'filth' is the definition.
(I've seen this before)
'floor regularly covered by endless old-fashioned' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'covered by' is an insertion indicator.
'endless' means to remove the last letter.
'old fashioned' becomes 'square'.
The alternating letters of 'floor' are 'lo'.
'square' with its final letter taken away is 'squar'.
'lo' placed inside 'squar' is 'SQUALOR'.
(Other definitions for squalor that I've seen before include "State of poverty and neglect" , "Filthiness" , "Disgusting filth" , "Sleaziness, mean poverty" , "Verdi's Egyptian opera" .)