No sitter, is ill-at-ease (6)
I believe the answer is:
uneasy
'ill-at-ease' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for uneasy that I've seen before include "Anxious, nervous" , "as a crowned head, proverbially" , "Restless, perturbed" , "Anxious and unsure, restless" , "Shakespeare: '. . . . . lies the head that wears a crown'" .)