Ring Kitty returned (4)
I believe the answer is:
loop
'ring' is the definition.
(a loop is a ring of material)
'kitty returned' is the wordplay.
'kitty' becomes 'pool' (synonyms).
'returned' says the letters should be written backwards.
'pool' backwards is 'LOOP'.
(Other definitions for loop that I've seen before include "Shape produced by curve that crosses itself" , "Aerobatics manoeuvre" , "Noose" , "Aerobatic manoeuvre" , "the knowledgeable are in it" .)