Alligator is one regarding soft floor covering (7)
I believe the answer is:
reptile
'alligator is one' is the definition.
'regarding soft floor covering' is the wordplay.
'regarding' becomes 're'.
'soft' becomes 'p' (musical abbreviation for 'piano').
'floor covering' becomes 'tile'.
're'+'p'+'tile'='REPTILE'
(Other definitions for reptile that I've seen before include "Cold-blooded vertebrate" , "Cold-blooded type" , "Cold-blooded beast" , "Snake or crocodile, say" , "eg Crocodile" .)