Rather upset, sit with dean (7)
I believe the answer is:
instead
'rather' is the definition.
(I know that rather can be written as instead)
'upset sit with dean' is the wordplay.
'upset' indicates anagramming the letters.
'with' says to put letters next to each other.
'sit' put after 'dean' is 'deansit'.
'deansit' with letters rearranged gives 'INSTEAD'.
(Other definitions for instead that I've seen before include "for a change" , "Stained (anag)" , "use this as a replacement" , "In lieu" , "As a substitute" .)