Brooding in a chair (7)
I believe the answer is:
sitting
'brooding' is the definition.
(thesaurus)
'in a chair' is the wordplay.
I cannot quite see how this works, but
'in' is within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sitting that I've seen before include "Session of spiritualists or court" , "On a seat" , "such an easy target" , "Well placed" , "Being portrayed" .)