Yet tin is changed for essence (6)
I believe the answer is:
entity
'for essence' is the definition.
(I've seen this in another clue)
'yet tin is changed' is the wordplay.
'is changed' indicates anagramming the letters.
'yet'+'tin'='yettin'
'yettin' anagrammed gives 'ENTITY'.
(Other definitions for entity that I've seen before include "Essence, existence" , "Something with independent existence" , "It exists as a seperate thing (6)" , "A thing with distinct and independent existence" , "Something existing in its own right" .)