Dog number ten, a rare item (5)
I believe the answer is:
curio
'a rare item' is the definition.
(a curio is a rare or unusual item)
'dog number ten' is the wordplay.
'dog' becomes 'cur' (both can mean a despicable person).
'number ten' becomes 'IO' (IO resembles 10).
'cur'+'io'='CURIO'
(Other definitions for curio that I've seen before include "'Rare, unusual object (5)'" , "Unusual, intriguing object" , "something odd" , "Bygone" , "Small fascinating collectible object" .)