Deters flu treatment, unsettled (9)
I believe the answer is:
flustered
'unsettled' is the definition.
(thesaurus)
'deters flu treatment' is the wordplay.
'treatment' indicates an anagram.
'deters'+'flu'='detersflu'
'detersflu' is an anagram of 'FLUSTERED'.
(Other definitions for flustered that I've seen before include "Hot and bothered, rattled" , "In quite a state, put out" , "Agitated, confused" , "Hot and bothered, rattles" .)