Endlessly sick patient treated getting clean (10)
I believe the answer is:
antiseptic
'clean' is the definition.
('antiseptic' is associated in meaning with 'clean')
'endlessly sick patient treated' is the wordplay.
'endlessly' means to remove the last letter.
'treated' is an anagram indicator.
'sick' with its final letter taken off is 'sic'.
'sic'+'patient'='sicpatient'
'sicpatient' is an anagram of 'ANTISEPTIC'.
'getting' is the link.
(Other definitions for antiseptic that I've seen before include "Preventing growth of, eg, bacteria" , "Clinical" , "Designed to counteract infection" , "Preventing growth of microorganisms" , "germicide" .)