Thus treated and pickled (6)
I believe the answer is:
soused
'pickled' is the definition.
(souse can mean to preserve in pickle)
'thus treated' is the wordplay.
'thus' becomes 'so' (synonyms).
'treated' becomes 'used' (using is a kind of treating).
'so'+'used'='SOUSED'
'and' acts as a link.
(Other definitions for soused that I've seen before include "Drenched with liquid" , "Soaked, pickled (gherkin); drunk (colloq.)" , "Marinated" , "Drunk or immersed in liquid" .)