Sameness of tiny diet (8)
I believe the answer is:
identity
'sameness' is the definition.
(I know that identity is a type of sameness)
'of tiny diet' is the wordplay.
'of' indicates anagramming the letters (the answer comes from or 'of' the letters).
'tiny'+'diet'='tinydiet'
'tinydiet' anagrammed gives 'IDENTITY'.
(Other definitions for identity that I've seen before include "Uniqueness" , "One's individuality" , "Specified name of description of tin deity" , "Sameness - individuality" , "Condition of being a specified person" .)