Ultimately you auction Dali off to our museum -- Saatchi not happy with lack of diversity (10)
I believe the answer is:
uniformity
'lack of diversity' is the definition.
(lack of variety or difference)
'ultimately you auction dali off to our museum' is the wordplay.
'ultimately' says to take the final letters.
'museum' becomes 'mity' (I am not sure about this - if you are sure you should believe this answer much more).
The last letters of 'you auction dali off to our' are 'unifor'.
'unifor'+'mity'='UNIFORMITY'
'saatchi not happy with' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
(Another definition for uniformity that I've seen is " absence of change".)