Fine extension: hide collapsed floor (4)
I believe the answer is:
fell
'hide collapsed floor' is the definition.
'fell' can be an answer for 'hide' (both can refer to an animal's hide). I am unsure of the 'collapsed floor' bit.
'fine extension' is the wordplay.
'fine' becomes 'f' (abbreviation).
'extension' becomes 'ell' (ell is a kind of extension).
'f'+'ell'='FELL'
(Other definitions for fell that I've seen before include "unloveable doctor" , "stretch of moorland" , "Came down - cut down" , "Collapsed; hill" , "Abated" .)