Unidentified object sat with rubbish (7)
I believe the answer is:
whatsit
'unidentified object' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'sat with rubbish' is the wordplay.
'rubbish' indicates anagramming the letters (a poor quality or rubbish spelling of the letters).
'sat'+'with'='satwith'
'satwith' is an anagram of 'WHATSIT'.
Can you help me to learn more?
(Other definitions for whatsit that I've seen before include "Thingummy" , "Something one can't recall" , "person not to be named" , "something indefinable" , "Thingumajig" .)