Possibly deem a sorceress (5)
I believe the answer is:
medea
'sorceress' is the definition.
(I've seen this before)
'possibly deem a' is the wordplay.
'possibly' indicates an anagram.
'deem' anagrammed gives 'mede'.
'mede'+'a'='MEDEA'
(Other definitions for medea that I've seen before include "Greek sorceress" , "In Greek myth, Jason's wife who killed their children in jealous rage" , "fabulous princess" , "Tragic heroine" , "Sorceress who helped Jason get the Golden Fleece" .)