Cleansing starch is an unnamed variation (9)
I believe the answer is:
catharsis
'cleansing' is the definition.
(catharsis is a kind of cleansing)
'starch is an unnamed variation' is the wordplay.
I cannot quite understand how this works, but
'is' is within the answer.
'an' could be 'a' and 'a' is present in the answer.
an anagram of 'starch' is 'cthars' which is present in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for catharsis that I've seen before include "Purification of the emotions" , "The release of pent-up emotions" , "purging process" , "relief of tensions" , "Release of feelings" .)