Vermin returned to Sirius, say (4)
I believe the answer is:
star
'to sirius say' is the definition.
(sirius is a kind of star)
'vermin returned' is the wordplay.
'vermin' becomes 'rats' (I've seen this before).
'returned' says the letters should be written in reverse.
'rats' reversed gives 'STAR'.
(Other definitions for star that I've seen before include "top player" , "Brilliant performer?" , "Leading actor in a show" , "Polaris, possibly" , "Famous entertainer" .)