Dog with one old trinket (5)
I believe the answer is:
curio
'trinket' is the definition.
(thesaurus)
'dog with one old' is the wordplay.
'dog' becomes 'cur' (a cur is an aggressive or scruffy dog).
'with' says to put letters next to each other.
'one' becomes 'i' (Roman numeral).
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'cur'+'i'+'o'='CURIO'
(Other definitions for curio that I've seen before include "Odd, unusual object" , "bit of bric-a- brac" , "Object that is interesting, being rare or unusual" , "Intriguing object worth collecting" , "Rare and unusual item" .)