Cover regularly left on fish (6)
I believe the answer is:
carpet
'cover' is the definition.
(carpeting is a kind of covering)
'regularly left on fish' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'on' means one lot of letters go next to another.
'fish' becomes 'carp' (carp is a kind of fish**).
The alternating letters of 'left' are 'et'.
'et' put after 'carp' is 'CARPET'.
(Other definitions for carpet that I've seen before include "Floor covering of heavy fabric" , "has pull up sharply" , "Lay into" , "Large rug" , "A red ...... is rolled out for VIPs" .)