In Tempest, Prospero's a tyrant (9)
I believe the answer is:
oppressor
'a tyrant' is the definition.
(thesaurus)
'in tempest prospero's' is the wordplay.
'in tempest' indicates an anagram (I've seen this in other clues).
'prosperos' is an anagram of 'OPPRESSOR'.
(Other definitions for oppressor that I've seen before include "Despot" , "One who keeps another in subjection" , "Proposers for one who persecutes" , "One tyrannises" , "Tyrant" .)