Superfluous transformed into leanness (12)
I believe the answer is:
nonessential
'superfluous' is the definition.
(thesaurus)
'transformed into leanness' is the wordplay.
'transformed' is an anagram indicator.
'into'+'leanness'='intoleanness'
'intoleanness' anagrammed gives 'NONESSENTIAL'.
(Other definitions for nonessential that I've seen before include "Not totally necessary" , "Dispensable" , "Not absolutely necessary" , "Not required" .)