Still in a terrible fluster (7)
I believe the answer is:
restful
'still' is the definition.
(thesaurus)
'terrible fluster' is the wordplay.
'terrible' indicates an anagram.
'fluster' anagrammed gives 'RESTFUL'.
'in a' acts as a link.
(Other definitions for restful that I've seen before include "Soothing is required" , "Still - calming" , "Calm" , "Relaxing" , "Quiet, soothing" .)