Object in tin, yet rotten (6)
I believe the answer is:
entity
'object in' is the definition.
'entity' can be an answer for 'object' (I've seen this before). I'm unsure of the 'in' bit.
'tin yet rotten' is the wordplay.
'rotten' indicates an anagram.
'tin'+'yet'='tinyet'
'tinyet' anagrammed gives 'ENTITY'.
Can you help me to learn more?
(Other definitions for entity that I've seen before include "Essence, existence" , "Something with independent existence" , "A thing with distinct and independent existence" , "Object" , "Anything that exists" .)