Room at the top for swinging cat around it backwards (5)
I believe the answer is:
attic
'room at the top' is the definition.
(room at the top of a house)
'swinging cat around it backwards' is the wordplay.
'swinging' indicates anagramming the letters.
'around' indicates putting letters inside.
'backwards' shows that the letters should be reversed in order.
'it' written backwards gives 'ti'.
'cat' with letters rearranged gives 'atc'.
'atc' going around 'ti' is 'ATTIC'.
'for' acts as a link.
(Other definitions for attic that I've seen before include "Loft room" , "Area at the top of the house" , "room with lots of junk?" , "Small room at the top" , "Ancient Greek; loft" .)