Cast part in Spellbound (4)
I believe the answer is:
rapt
'spellbound' is the definition.
(thesaurus)
'cast part' is the wordplay.
'cast' indicates an anagram.
'part' with letters rearranged gives 'RAPT'.
'in' is the link.
(Other definitions for rapt that I've seen before include "Part being transported" , "Part (anag.)" , "Fascinated in part" , "Completely absorbed" , "Held spellbound" .)